Glossary of eye are terms - A-Z

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Understanding eye care terminology can make it easier to discuss your eye care and vision needs with your eye care provider. This glossary provides brief definitions of common terms related to eye health, vision, eyewear and more, organized alphabetically.

Abbe value

Abbe value is a measure of the tendency of an eyeglass lens material to cause chromatic aberration, which is an optical distortion that causes colored halos or fringes around lights or blurred vision, especially when looking through the periphery of the lens. The lower the Abbe number, the more likely chromatic aberration will be noticeable.

Aberrometer

An aberrometer is a device that can identify common and more obscure vision errors by measuring the way light waves travel through the eye's optical system.

Ablation

Ablation is a term that means “removal.” In vision, ablation refers to the surgical removal of eye tissue to correct a refractive error, such as myopia. For example, in laser procedures like LASIK and PRK, the excimer laser ablates, or removes, tissue from the cornea.

Acanthamoeba keratitis

Acanthamoeba keratitis is a rare but serious eye infection caused by Acanthamoeba, a single-celled organism found in water and soil. It primarily affects contact lens wearers, particularly those who swim, shower, or use tap water while wearing lenses. Symptoms include severe eye pain, redness, light sensitivity, blurred vision, and a feeling of something in the eye. Prompt diagnosis and prolonged treatment with specialized antimicrobial drops are essential to prevent vision loss.

Accommodation

Accommodation refers to the eye's ability to automatically change focus from seeing at one distance to seeing at another.

Accommodation disorder

Accommodation disorders have a variety of causes. Symptoms include blurred vision, double vision, eye strain, headache, fatigue and difficulty concentrating (particularly while reading). Presbyopia is an accommodation disorder that affects nearly everyone as they get older, since its causes relate to the aging of the eyes.

Acetate

Acetate is a type of plastic often used in eyeglass frames.

Acrodermatitis enteropathica

This is a rare inherited condition characterized by dermatitis, hair loss, diarrhea and zinc deficiency. One of the more common eye symptoms is light sensitivity, but conjunctivitis and other signs and symptoms may also be present.

Age-related macular degeneration (AMD)

Age-related macular degeneration (AMD) is a leading cause of central vision loss in adults over 50, caused by damage to the macula — the central part of the retina responsible for sharp, detailed vision. AMD has two main forms: dry AMD, in which the macula gradually thins and yellow deposits called drusen form, and wet (neovascular) AMD, in which abnormal blood vessels grow under the retina and leak fluid or blood. Risk factors include age, family history, smoking, and cardiovascular disease.

Albinism

Albinism is a condition where a person or animal lacks pigment. The eyes often have very light blue or pink irises and a pink pupil (due to a lack of pigment inside the back of the eye). Visual signs and symptoms can include light sensitivity, nystagmus, blurred vision, vision loss and strabismus.

Allergen

An allergen is any substance (such as pollen, mold, dust or animal dander) that causes allergy symptoms in sensitive individuals.

Allergic conjunctivitis

Allergic conjunctivitis is inflammation of the conjunctiva caused by an allergic reaction to substances such as pollen, dust mites, pet dander or mold. Signs and symptoms typically include itching in both eyes (the hallmark feature — usually in the corners of the eyes toward the nose), redness, watery discharge, chemosis (conjunctival swelling) and eyelid puffiness. Both eyes are usually affected. Seasonal allergic conjunctivitis occurs in response to airborne allergens at certain times of the year, while perennial allergic conjunctivitis can occur year-round. Treatment options can include allergen avoidance, cool compresses, artificial tears, antihistamine or mast cell stabilizer eye drops and, in some cases, oral antihistamines.

Allergy

An allergy is a reaction of the body's immune system to a foreign substance (e.g., pollen, animal dander, etc.). When the eyes are affected, the more common signs and symptoms are often redness, itching, chemosis, tearing, swollen eyelids and stickiness. Read more about allergies and the eyes.

Amsler grid

The Amsler grid is a square pattern of horizontal and vertical lines with a central dot, used to detect changes in central vision. Patients are asked to look at the central dot one eye at a time and report whether any lines appear wavy, missing or distorted. The grid is commonly used to monitor macular conditions such as age-related macular degeneration.

Amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis, also called Lou Gehrig’s disease, is a disease that causes the degeneration of neurons in the brain stem and spinal cord. Voluntary muscles atrophy and can eventually become paralyzed.

Amaurosis fugax

Amaurosis fugax is the sudden and usually temporary vision loss caused by an "eye stroke." When a clot or blockage disrupts blood flow to the eye, symptoms can include curtain-like darkness, usually in just one eye.

American National Standards Institute (ANSI)

The American National Standards Institute is a private, non-profit organization that coordinates efforts to develop standards for manufacturing many different products, including eyeglass lenses. For example, certain ANSI standards define acceptable levels of impact resistance for safety eyewear.

Amblyopia

Amblyopia, also called lazy eye, is a condition in which central vision does not develop properly in one eye, leading to the use of the other eye as the dominant eye. Strabismus is the leading cause, followed by anisometropia. There are typically no symptoms. The patient may be found squinting and closing one eye to see. There may be unrecognized blurred vision in one eye and vision loss.

Angiogenesis

Angiogenesis is the formation of new blood vessels in the body.

Aniridia

Aniridia refers to an absent or partially absent iris and is typically congenital. Additional symptoms can include poor vision and photophobia (light sensitivity).

Anisocoria

Anisocoria refers to unequal pupil size. Causes can include glaucoma, head or eye trauma, an intracranial tumor, infection of the membranes surrounding the brain and previous intraocular surgery. A small percentage of the population has unequally sized pupils naturally (without any known cause).

Anisometropia

Anisometropia is a condition where the eyes have a significantly different refractive power from each other, so the prescription required for clearer vision will be different for each eye.

Anophthalmos

Anophthalmos refers to the absence of one or both eyes. It may be congenital or due to trauma, infection or other causes. Symptoms can include reduced depth perception and peripheral vision.

ANSI Z87.1-2003 Standard

The ANSI Z87.1-2003 standard is the American National Standards Institute’s (ANSI) Practice for Occupational and Educational Eye and Face Protection. Eyewear that meets this standard is generally considered safer than eyewear that does not.

Anterior chamber

The anterior chamber is the part of the eye behind the cornea and in front of the iris and lens.

Antibody

An antibody is a protein activated by the body's immune system that fights infection, toxins and other foreign substances.

Antioxidant

An antioxidant is a substance that helps protect cells from damage caused by free radicals — unstable molecules that can contribute to aging and disease. Common dietary antioxidants include vitamin C, vitamin E, beta-carotene, and the carotenoids lutein and zeaxanthin. The AREDS2 study established that a specific combination of vitamin C, vitamin E, lutein, zeaxanthin, zinc and copper can reduce the risk of progression in people with certain eye diseases such as intermediate dry AMD or advanced age-related macular degeneration.

Anti-reflective (AR) coating

Anti-reflective coating is a thin layer(s) applied to a lens to reduce the amount of reflected light and glare that reaches the eye.

Anti-VEGF (anti-vascular endothelial growth factor)

Anti-VEGF (anti-vascular endothelial growth factor) medications are drugs that block the activity of VEGF, a protein that promotes abnormal blood vessel growth and leakage in the retina. Delivered as injections into the eye, anti-VEGF drugs are the standard treatment for wet age-related macular degeneration, diabetic macular edema, diabetic retinopathy and retinal vein occlusion. FDA-approved biosimilars to several of the anti-VEGF drugs are also available.

Apheresis

Apheresis is a medical procedure in which blood is drawn from the body, separated into components, and a specific component (such as plasma, platelets, or white blood cells) is removed before the rest of the blood is returned. Apheresis has many applications in medicine, including the collection of blood components for transfusion and the removal of harmful substances in certain conditions.

Aqueous humor

Aqueous humor is the clear, watery fluid that fills the space between the cornea and the natural crystalline lens of the eye. The aqueous humor (or aqueous) is produced by the ciliary body behind the iris and drains from the anterior chamber of the eye through the trabecular meshwork Failure of the aqueous humor to drain properly from the eye increases intraocular pressure (IOP) and can lead to glaucoma.

Argyll Robertson pupil

This is a small, irregular pupil, usually caused by syphilis. Argyll Robertson pupils do not respond to light.

Aspheric

Aspheric is a term used to describe a corrective lens with a gradually changing curvature rather than a uniform curvature across its surface. Aspheric eyeglass lenses are commonly prescribed for people who have strong vision prescriptions because they are typically thin and lightweight and can reduce distortion and eye magnification. Aspheric contact lenses can work as a multifocal lens, or to correct a single-vision condition like astigmatism.

Asteroid hyalosis

Asteroid hyalosis is a benign condition that creates suspended particles within the eye's interior, observable by an eye doctor during an eye exam. These yellowish particles made of fats (lipids) rarely interfere with vision or cause symptoms. The particles move within the eye's gel-like vitreous and resemble stars at night. (Floaters are not usually associated.) The condition is not well understood, but it appears to be associated with aging.

Astigmatic keratotomy (AK)

Astigmatic keratotomy is a procedure in which an eye surgeon makes an incision in the cornea so that it is more spherical when it heals, thus reducing astigmatism.

Astigmatism

Astigmatism is a refractive error where objects at all distances have a distorted appearance (such as "streaked" or stretched, with elongated shapes and unequally blurred borders). Signs and symptoms of astigmatism can include squinting, eye strain, headaches and difficulty reading.

Atopy

Atopy is a type of allergy where levels of the antibody immunoglobulin E are increased. Atopy includes rhinitis, asthma, hay fever and eczema.

Aura

An aura is a sensation that may be experienced before an attack of epileptic seizure, migraine or other disorder. Examples are flashes of light, colored lights, numbness, coldness and even hearing voices.

Aviator glasses

These are glasses with a large, upside-down teardrop shape like the sunglasses that aircraft pilots have traditionally worn. They commonly have a metal frame and large, tinted lenses, but they are also available with plastic frames and clear lenses. Often, they have a double-bar bridge.

Band keratopathy

Band keratopathy is an opacity of the corneal stroma and Bowman's membrane. Symptoms can include vision loss and foreign body sensation.

Base curve

The term base curve refers to the main or steepest part of the curve used in the design of a contact lens. Measured in millimeters, the base curve is prescribed to match or complement the curvature of your cornea. The lower the number, the steeper the curve of your cornea.

Bell's palsy

Bell's palsy is a sudden, usually temporary weakness or paralysis of the muscles on one side of the face, caused by inflammation of the facial (seventh cranial) nerve. The cause is most often unknown, though it has been associated with viral infections. Because the affected eyelid may not fully close, patients can develop dryness, irritation and exposure of the cornea, and may need lubricating drops, ointment, or eyelid taping during recovery. Most cases improve over weeks to months.

Benign essential blepharospasm

This is a neurological disorder causing involuntary muscle contractions, with eyelid muscle spasms.

Best corrected visual acuity (BCVA)

This refers to the sharpest vision you can achieve with correction (such as glasses), as measured on the standard Snellen eye chart. For example, if your uncorrected eyesight is 20/200, but you can see 20/20 with glasses, your BCVA is 20/20.

Best disease

Best disease, also called Best vitelliform macular dystrophy, is an inherited disorder that causes a yellow, yolk-like deposit to form in the macula, typically appearing in childhood or adolescence. The condition can lead to slow, progressive loss of central vision, although many people retain useful vision into adulthood. There is no cure, but low-vision support and monitoring are mainstays of care.

Beta blocker

Beta blockers are drugs that block the action of adrenaline at beta-adrenergic receptors, which slows heart rate and lowers blood pressure. As eye drops, beta blockers lower intraocular pressure by reducing aqueous humor production and are used in the treatment of glaucoma. Side effects can include slowed heart rate, fatigue and breathing problems, particularly in people with asthma or COPD.

Beta titanium

Beta titanium is a titanium alloy material used to make eyeglass frames. Beta-titanium eyeglass frames are often both strong and flexible.

Bifocal

A bifocal lens has one segment for near vision and one segment for far vision. The term "bifocal" can apply to both eyeglass lenses and contact lenses.

Binocular vision

Binocular vision refers to the ability of both eyes to work together to achieve proper focus, depth perception and range of vision.

Biocompatible

The term biocompatible refers to the ability to coexist with living tissues without causing harm. For example, artificial lenses are designed to be biocompatible with the tissue inside the eye, reducing the risk of a toxic or immunological response that could harm the eye.

Biomimetic

The term biomimetic refers to synthetic products or systems designed to mimic the structure and function of natural or biological systems, such as those found in the body. For example, retinal implants are designed to imitate how the retina processes light signals and transmits this information to the brain.

Bioptic telescopic lenses (BTL)

Bioptic telescopic lenses are devices attached to glasses that can provide heightened magnification, typically used for driving. For people with low vision who qualify, telescopic lenses may be attached above the driver's line of sight to help magnify objects such as road signs.

Bleb

In eye care, a bleb is a small, blister-like elevation of the conjunctiva that forms after certain glaucoma surgeries, such as trabeculectomy. The bleb covers a surgically created drainage channel that allows aqueous humor to flow out of the eye, lowering intraocular pressure. Blebs require periodic monitoring for leaks or infection (blebitis).

Blepharitis

Blepharitis is a condition characterized by inflammation of the eyelid(s), typically around the eyelashes. Various types of dermatitis, rosacea and allergic reactions can cause blepharitis. There are two types: anterior and posterior. Signs and symptoms often include red or pink eyelids, crusty lids or lashes, burning, foreign body sensation, eye or eyelid pain or discomfort, dry eyelid, dry eye, eyelash loss, grittiness, stickiness, eyelid swelling and tearing.

Blepharochalasis

Blepharochalasis is a condition characterized by excessive, drooping eyelid skin caused by recurring swelling. This condition typically occurs in young people.

Blepharoconjunctivitis

Blepharoconjunctivitis is a condition characterized by inflammation of the eyelid and conjunctiva. Infections and allergic reactions are among the causes. Signs and symptoms can include a red or pink eye, a red or pink eyelid, pain or discomfort of the eye or around the eye, tearing, burning, eye dryness and eye stickiness.

Blepharoplasty

Blepharoplasty is a surgical procedure to reshape the upper or lower eyelids, typically by removing excess skin, muscle or fat. It may be performed for cosmetic reasons or, when drooping eyelid skin (dermatochalasis) interferes with the visual field, for functional reasons.

Blepharospasm

The term blepharospasm refers to involuntary increased blinking that progresses to spasms in both eyes. The exact cause is unknown, but doctors believe it to be a central nervous system disorder. It can potentially lead to functional blindness in cases where a patient can't open their eyes long enough to function visually.

Blue light

Blue light is the portion of the visible light spectrum with the shortest wavelengths, sitting next to ultraviolet light. It is sometimes called High Energy Visible (HEV) light. Blue light is emitted by the sun as well as by digital screens, LED lighting and fluorescent bulbs. Exposure to blue-violet light from screens, particularly in the evening, may disrupt sleep by affecting melatonin production. Blue light glasses may help improve visual comfort by reducing exposure to blue-violet light. Current evidence does not support a link between blue light and permanent eye damage or age-related macular degeneration.

Botulism

Botulism is a serious illness from a toxin produced by Clostridium bacteria (usually Clostridiumbotulinum). Infant botulism and food-borne botulism are the most common forms in the United States. Signs and symptoms can include double vision, blurred vision, drooping eyelids (ptosis), muscle weakness, difficulty speaking, difficulty swallowing, difficulty breathing and nausea.

Bowman's membrane

Bowman’s membrane is the corneal layer between the epithelium and the stroma.

Bridge

This term refers to the part of an eyeglass frame that extends across the nose.

C-D

Cable temple

A cable temple is a type of "arm" on a pair of glasses that wraps around the ear to help keep the eyewear firmly in place. It is often used in eyeglasses for infants and toddlers.

Canaliculus

A canaliculus is a tiny, tube-shaped passageway in the body. In the eye, canaliculi are tiny channels located at the beginning of the tear ducts, through which tears drain until they exit into the nose.

Canaliculitis

Canaliculitis is inflammation of a tear duct (or ducts), often caused by a bacterial infection. Signs and symptoms of this condition can include discharge, a red or pink eye, and a swollen upper or lower eyelid near the nose.

Capsulotomy

In cataract surgery, an incision is made in the capsule that contains the natural crystalline lens of the eye to remove and replace the natural lens with an artificial one.

Carotenoid

A carotenoid is a pigmented substance that adds color, such as red, orange, or yellow, to plants. Carotenoids have antioxidant properties that help protect cells against damage from free radicals, which are unstable atoms that can interact with and break down healthy tissue in different parts of the body, including the eyes.

Cataract

A cataract is the clouding of the eye’s natural lens, which sits behind the iris and helps focus light onto the retina. Cataracts most often develop with age but can also be related to UV exposure, smoking, long-term steroid use, diabetes, eye injury or be present at birth. Symptoms include blurry or hazy vision, glare, halos around lights, faded color perception and sometimes a temporary shift toward nearsightedness (“second sight”). Cataracts are treated with surgery, in which the clouded natural lens is removed and replaced with an artificial intraocular lens (IOL).

Cavernous sinus problem

The cavernous sinus is located at the base of the cranium and contains the carotid artery and cranial nerves. Problems in the cavernous sinus can include tumors, aneurysms and clots. Typical signs and symptoms include ophthalmoplegia, chemosis and a bulging eye. You may also experience eye redness and vision loss.

Central island

This is a refractive surgery complication in which the laser leaves an "island" of corneal tissue in the concave ablation zone. Symptoms include double vision and distortion. Learn more about the potential complications of LASIK and other refractive surgeries.

Central serous retinopathy (also called central serous chorioretinopathy)

Central serous retinopathy (CSR) is a condition in which fluid collects under the central retina (macular area) and disrupts central vision. The cause is unknown. Symptoms can include blurred central vision and metamorphopsia. Some patients also develop floaters.

Cerebral cortex

The cerebral cortex is the outer portion of the brain where complex functions, including certain vision processes, take place.

Cerebral palsy

Cerebral palsy is a group of chronic conditions caused by brain damage before birth or during infancy. The many effects can include poor muscle coordination/control, muscle weakness/tightness, involuntary movements, difficulty with swallowing, speech problems, seizures, sensory impairment, learning disabilities and mental retardation. No cure exists, but training and therapy can improve function and quality of life.

Chalazion

A chalazion is a small bump on the eyelid caused by an obstructed meibomian gland. Additional signs and symptoms can include light sensitivity, tearing and eyelid swelling. Chalazia are usually not painful unless they become infected.

Charles Bonnet syndrome

Charles Bonnet syndrome is a condition in which people with significant vision loss experience vivid visual hallucinations without any underlying psychiatric illness. The hallucinations can range from simple patterns and shapes to detailed images of people, animals or scenes, and patients typically recognize them as not real. It is most common in older adults with conditions such as age-related macular degeneration, glaucoma or diabetic retinopathy. There is no specific treatment, but reassurance that the hallucinations are a known response to reduced visual input — not a sign of mental illness — is an important part of care.

Chemosis

Chemosis is conjunctival swelling that is often caused by an allergy.

Choroid

The choroid is the layer of blood vessels located between the sclera (white of the eye) and the retina. It helps provide nourishment to the back portion of the eye.

Choroidal neovascularization

Choroidal neovascularization is the abnormal growth of new blood vessels in the choroid. It is commonly associated with macular degeneration, but it can occur as a result of other eye conditions as well. Symptoms can include vision loss and metamorphopsia.

Choroidal nevus

A choroidal nevus is a benign, freckle-like pigmented spot in the choroid, the vascular layer of the eye beneath the retina. Most choroidal nevi cause no symptoms and are found incidentally during a dilated eye exam. They are monitored periodically because, in rare cases, a nevus can transform into a choroidal melanoma.

Chromosomes

Chromosomes are paired strands of DNA that contain genes or inherited traits. Each normal cell in the human body typically has 23 pairs of chromosomes, 46 altogether, with one half contributed by the mother and the other half contributed by the father.

Ciliary body

The ciliary body is the part of the eye located between the iris and the choroid, which, together, form the uvea. The ciliary body's main functions are accommodation, aqueous humor production and holding the lens in place.

Clip-on

This is a type of sunglass lens that attaches to your regular eyeglasses.

Coats disease

Coats disease is a rare, usually one-sided eye disorder in which abnormal blood vessels in the retina leak fluid and lipid, causing retinal swelling and detachment. It most often affects boys in the first decade of life. Symptoms include decreased vision, strabismus or a white pupillary reflex (leukocoria). Early diagnosis allows for laser, cryotherapy or anti-VEGF treatment to preserve vision.

Cohort

This is a term used in clinical studies to define a set of people who have something in common, such as similar backgrounds, experiences or health problems.

Collagen

Collagen is a fibrous protein found in bones and connective tissue; it is also present in the eye. One type of vision correction surgery heats collagen around the edges of the cornea (which lets light into the eye). This procedure reshapes the cornea, helping it focus light right onto the retina, for clearer vision.

Coloboma

A coloboma is a cleft, usually due to incomplete embryologic development in utero. An iris coloboma is the more common type of eye coloboma; the pupil will often look like a keyhole or upside-down pear. Colobomas can also affect other eye structures, such as the eyelid, retina and optic nerve. However, only iris and eyelid colobomas are visible to the naked eye. Additional symptoms, such as poor vision, may occur but may not be apparent from a parent's perspective.

Color blindness

Color blindness is the partial or total inability to distinguish specific colors. Genetics is the more common cause, but it can also result from certain conditions, some medications, injuries and aging. It is more common in men than in women.

Conductive keratoplasty (CK)

Conductive keratoplasty is a refractive procedure that uses radio-frequency energy to heat and shrink collagen in the peripheral cornea, steepening its central curvature to reduce hyperopia or presbyopia. CK was FDA-approved in the early 2000s but is rarely performed today; it has largely been replaced by other refractive surgery options.

Cone

A cone is a type of photosensitive cell in the retina that helps you to see color.

Conjunctiva

The conjunctiva is the thin, normally clear, moist membrane that covers the white part of the eye (sclera) and the inner surface of the eyelids.

Conjunctivitis

Conjunctivitis is inflammation of the conjunctiva, characterized by a pink eye. It can be caused by bacteria, viruses, allergens or irritants, though the term "pink eye" is commonly used for the infectious types of conjunctivitis. Other signs and symptoms can include burning, discharge, dryness, itching, light sensitivity, eye pain or discomfort, stickiness, tearing and chemosis.

Contact lens drops

These are eye drops specifically designed for contact lens wearers; regular eye drops can discolor contact lenses.

Contact lens problem

Contact lens problems can range from minor to sight-threatening, and include protein build-up, debris on the lens, a ripped or nicked lens, infections and more. Signs and symptoms can include frequent blinking, blurred vision, burning, discharge, foreign body sensation, itching, light sensitivity, eye pain or discomfort, a red or pink eye or lid and eyelid swelling.

Contrast sensitivity

Contrast sensitivity is the ability of the eye to detect the line of demarcation between an object and its background or an adjacent object.

Convergence

Convergence is the eye’s ability to turn inward. People with convergence insufficiency may have difficulty (eye strain, blurred vision, etc.) with near tasks, such as reading.

Convergence insufficiency

Convergence insufficiency is a binocular vision disorder in which the eyes have difficulty turning inward together to focus on a near object. Symptoms typically appear during reading or close work and include eye strain, double vision, blurred vision, headaches and loss of place on the page. It is treated with office-based and at-home vision therapy and, in some cases, prism lenses.

Cornea

The cornea is the clear, dome-shaped portion of the front surface of the eye that allows light to enter the eye for sight. The cornea provides the majority of the focusing power of the eye.

Cornea verticillata

Cornea verticillata is a whorl-like pattern of fine, golden-brown or grayish deposits in the corneal epithelium, visible during a slit-lamp exam. It is a known side effect of several medications, most commonly amiodarone, and is also a hallmark of Fabry disease. The deposits rarely affect vision and typically resolve when the offending medication is stopped.

Corneal abrasion

A corneal abrasion is a loss of the epithelial layer of the cornea, typically due to minor trauma, such as contact lens trauma, a sports injury, or damage from dirt or another foreign body. Signs and symptoms can include blurred vision, foreign body sensation, grittiness, light sensitivity, eye pain or discomfort, a red or pink eye and tearing.

Corneal arcus

This is a visible white or gray clouding of the peripheral cornea, caused by lipid (fat) deposits. The opacity can be arc-shaped or it can form a complete ring around the cornea. Corneal arcus in older adults is sometimes called arcus senilis. Corneal arcus in younger individuals can be associated with high cholesterol.

Corneal collagen crosslinking

Corneal collagen crosslinking is a procedure that involves the application of riboflavin (vitamin B2) eye drops and ultraviolet light to the eye to strengthen corneal tissue in an eye affected by keratoconus.

Corneal dystrophy

Corneal dystrophy is one of a group of conditions, usually hereditary, in which the cornea loses its transparency. The corneal surface is no longer smooth. Common forms include map-dot-fingerprint dystrophy, Fuchs' dystrophy and lattice dystrophy. Symptoms can include blurred vision, foreign body sensation, light sensitivity, eye pain or discomfort and vision loss.

Corneal ectasia

Corneal ectasia is the progressive thinning of the middle layer of the cornea (stroma), which causes the front surface of the eye to bulge forward. Ectasia can cause irregular astigmatism, higher-order aberrations, distorted vision and corneal scarring. Causes of corneal ectasia can include keratoconus, other corneal diseases and (infrequently) LASIK surgery. The condition is also called keratectasia or keratoectasia.

Corneal edema

Corneal edema refers to the swelling of the eye's cornea. It can be caused by intraocular surgery, corneal dystrophies, high intraocular pressure and contact lens complications, among other conditions and factors. Symptoms can include vision loss, halos around lights, a white or cloudy spot on the eye, photophobia, eye pain and foreign body sensation.

Corneal endothelium

This is the cornea's inner layer of cells.

Corneal epithelium

This is the cornea's outer layer of cells.

Corneal erosion

Corneal erosion is the recurrent breakdown of the corneal epithelium, typically caused by a previous corneal abrasion or by map-dot-fingerprint dystrophy. Symptoms can include blurred vision, foreign body sensation and eye pain or discomfort.

Corneal implants

Corneal implants are devices (such as ring segments or contacts) placed in the eye, usually to correct vision.

Corneal (intrastromal or allogenic) ring segment

This is a type of vision correction surgery where an eye doctor inserts a tiny plastic ring into the cornea (which lets light into the eye). This ring reshapes the cornea, helping it to focus light onto the retina properly so you can see more clearly. The ring can be adjusted and even removed if desired.

Corneal opacity

A corneal opacity is a cloudy spot in the cornea, which is normally transparent. Causes can include corneal scar tissue and infection. Symptoms can include halos around lights, photophobia, vision loss, and a white or cloudy spot on the eye.

Corneal reshaping therapy

This therapy involves the use of specially designed gas permeable (GP) lenses to reshape the cornea during sleep and temporarily correct myopia (with or without mild-to-moderate astigmatism).

Corneal stroma

The stroma is the cornea's middle layer. It consists of lamellae (collagen) and cells, and makes up most of the cornea.

Corneal topography

A corneal topographer shines light onto the surface of the eye, then measures the reflected light to create a map of the cornea's curvature as well as any irregularities. The map is commonly used for evaluations related to refractive surgery, contact lens fitting and corneal disease management. It can also be especially useful for measuring astigmatism. The color map uses blue and green to represent flatter areas of the cornea, while red and orange represent steeper areas.

Corneal ulcer

A corneal ulcer is an infected corneal abrasion. This condition commonly affects extended wear contact lens wearers. A corneal ulcer is an ocular emergency. Signs and symptoms can include light sensitivity, eye pain or discomfort, a red or pink eye, a white or cloudy spot on the eye and tearing.

Cranial nerve

The cranial nerves are 12 pairs of nerves that extend from the brain to other parts of the head. Those that affect the eyes and vision are the second cranial nerve (optic), third (oculomotor), fourth (trochlear), fifth (trigeminal), sixth (abducens) and seventh (facial). The optic nerve carries stimuli from the rods and cones to the brain. The third, fourth and sixth cranial nerves work with the eye muscles to control eye movement. The fifth cranial nerve provides sensory information to the eye, including the cornea and eyelids. The seventh cranial nerve works with the facial muscles to control facial movement (specifically the closure of the eyelids).

Cranial nerve palsy

Palsy (full or partial paralysis) of the third, fourth or sixth cranial nerves can result in difficulty moving the eye, causing signs and symptoms such as eyes that don't point in the same direction, reduced depth perception, double vision, ptosis, vision loss, a dilated pupil that doesn't respond to light and head tilting. Causes can include head trauma, diabetes, tumors, aneurysms, infarction (tissue death) and more. In many cases, the cause of paralysis of the seventh cranial nerve is unknown (termed "Bell's palsy"). Signs and symptoms of this condition can include weak facial muscles, difficulty closing the eye, infrequent blinking, earache, acute hearing, facial drooping, ectropion, tearing, eye dryness, blurred vision and a burning feeling in the eye.

Crossed eyes

This is a type of strabismus (a misalignment of the eyes) where one or both eyes point inward, toward the nose.

Crystalline lens

The crystalline lens is the clear, flexible, biconvex structure inside the eye, located behind the iris. It works with the cornea to focus light onto the retina, and changes shape (accommodation) to focus on near objects. With age, it loses flexibility (causing presbyopia) and can become cloudy (forming a cataract).

Culture

In medical terminology, a culture is a test of a sample, such as blood or other body material, to determine the presence of bacteria or other substances.

Cup-to-disc ratio (C/D ratio)

The cup-to-disc ratio is a measurement that compares the size of the optic cup (the central depression in the optic disc) to the overall diameter of the optic disc. Eye doctors use it to assess the health of the optic nerve. A larger or asymmetric cup-to-disc ratio, or an increase in the ratio over time, can be a sign of glaucoma. The ratio is typically monitored alongside intraocular pressure, optical coherence tomography (OCT) imaging, and visual field testing.

Cycloplegia

Cycloplegia is the paralysis of the eye's ciliary (focusing) muscle and power of accommodation. A cycloplegic refraction is performed after eye drops are placed in the eye to dilate the pupil and temporarily paralyze the accommodative ability of the eye, which might mask some degree of refractive error.

Cystoid macular edema (CME)

This is the swelling of the eye's macula, caused by an excessive amount of fluid.

Cytomegalovirus (CMV) retinitis

CMV retinitis is a serious eye infection that can affect people with immune conditions, such as AIDS. Symptoms can include floaters, blind spots, blurry vision and vision loss. 

Dacryoadenitis

Dacryoadenitis is inflammation of the tear gland, typically caused by a viral or bacterial infection. Signs and symptoms can include dry eye, a red or pink eyelid, swelling of the lid or tissue around the eyes, and ptosis.

Dacryocystitis

Dacryocystitis is inflammation of the nasolacrimal (tear) sac, typically caused by dacryostenosis. Signs and symptoms can include discharge, a sticky eye, eye pain or discomfort, a red or pink eye, swelling around the eye and tearing.

Dacryostenosis

Dacryostenosis is a blocked tear duct, which is characterized by excessive tearing. It can also cause eye discharge or stickiness.

Daily wear

These are a type of soft contact lenses worn every day for six months up to a couple of years. They require daily cleaning and disinfecting.

Decentered ablation

Also called decentration, this is a refractive surgery complication in which the laser is not centered on the pupil when it removes tissue. Symptoms can include glare, double vision and halos.

Demodex blepharitis

Demodex blepharitis is a form of eyelid inflammation caused by an overgrowth of Demodex mites, microscopic organisms that normally live in human hair follicles. Symptoms include itchy or irritated eyelids, redness along the lash line and “collarettes” — cylindrical, waxy debris at the base of the eyelashes. Treatment options include lid hygiene with tea tree oil–based products and prescription topical or oral therapies.

Dermatochalasis

Dermatochalasis is the excessive, drooping eyelid skin caused by a loss of elasticity in aging skin.

Descemet's membrane

This is the corneal layer between the stroma and the endothelium.

Descemet’s membrane endothelial keratoplasty (DMEK)/Descemet’s stripping automated endothelial keratoplasty (DSAEK)

DMEK (Descemet membrane endothelial keratoplasty) and DSAEK (Descemet stripping automated endothelial keratoplasty) are partial-thickness corneal transplant procedures that replace only the diseased innermost layers of the cornea, rather than the entire cornea. They are used to treat conditions affecting the corneal endothelium, such as Fuchs’ endothelial dystrophy. Compared with full-thickness corneal transplants, they offer faster visual recovery and lower risk of rejection.

Diabetic macular edema (DME)

Diabetic macular edema is swelling of the macula caused by leaky retinal blood vessels in people with diabetic retinopathy. It is the most common cause of vision loss in people with diabetes. Symptoms can include blurred, wavy or washed-out central vision. Treatment may include intravitreal anti-VEGF injections, steroid implants, focal laser, and tight control of blood sugar and blood pressure.

Diabetic maculopathy

Diabetic maculopathy is an eye disease related to diabetes that can cause swelling and abnormal leakage of fluids and fats into the macula, where fine focusing and central vision occur. Because of the location of eye damage, severe diabetic maculopathy can cause loss of central vision and blindness.

Diabetic retinopathy (DR)

Diabetic retinopathy (DR) is an eye disease caused by damage to the blood vessels of the retina from chronically elevated blood sugar. It is a leading cause of vision loss in working-age adults. In its earlier, non-proliferative stage, retinal blood vessels can leak, swell or close off. In its advanced, proliferative stage, fragile new blood vessels grow on the retina or optic nerve and can bleed into the vitreous or cause retinal detachment. Many people have no symptoms in early disease, which is why annual dilated eye exams are recommended for people with diabetes. Treatment options include strict blood sugar and blood pressure control, intravitreal anti-VEGF injections, focal or panretinal laser, and vitrectomy in advanced cases.

Diabetic vitreous hemorrhage

A diabetic vitreous hemorrhage occurs when blood enters the gel-like vitreous of the eye, originating from blood vessels weakened by diabetes. Because blood in the vitreous can block vision, a procedure called a vitrectomy may be required.

Digital eye strain

Digital eye strain (also called computer vision syndrome) refers to a group of eye and vision-related symptoms that result from prolonged use of digital screens. Common ocular symptoms include eye fatigue, dryness, irritation, burning, blurred vision and headaches. Associated symptoms can include neck, shoulder and back discomfort related to posture during screen use. Symptoms are usually temporary and improve with regular breaks, ergonomic adjustments and addressing any underlying refractive error.

Dilated eye exam

A dilated eye exam is a comprehensive eye exam in which eye drops are used to enlarge the pupils, allowing the eye doctor to see clearly into the back of the eye and evaluate the retina, optic nerve, and macula. It is the standard way to screen for and monitor many conditions, including diabetic retinopathy, glaucoma and macular degeneration.

Diode

A diode is a device, such as a semiconductor, that conducts electricity in one direction. A light-emitting diode (LED) can produce various wavelengths, colors and intensities of light.

Diopter (D)

A diopter is a unit of measure for the refractive (light-bending) power of a lens; eye care practitioners use it in both eyeglass prescriptions and contact lens prescriptions. A negative number refers to nearsightedness; a positive number, farsightedness. For example, someone with -8.00 diopter lenses is very nearsighted, while someone with +0.75 diopter lenses is only slightly farsighted.

Diplopia (double vision)

Diplopia occurs when two images of the same object are perceived by one or both eyes.

Disc hemorrhage

This is bleeding in the back of the eye, usually occurring at the edge of the optic disc (the area where the optic nerve attaches to the retina). It has several different names:  splinter/flame-shape/Drance hemorrhage. Though disc hemorrhages usually disappear in a few months, they are a serious condition, as they can be a sign of active glaucoma.

Disposable contact lenses

Technically, this is any contact lens that is thrown away after a short period of time. Among most eye care practitioners, "disposable" usage ranges from one day to two weeks, while "frequent replacement" lenses are discarded monthly or quarterly.

Distance vision

Distance vision generally refers to eyesight for tasks beyond arm's length, such as driving, watching television and movies, and participating in sports. Distance vision is tested during an eye exam with a standard eye chart at a distance of approximately 20 feet.

Dk/t

Dk is the oxygen permeability of a contact lens material; it is the thickness of the contact lens design. Dk/t is a measurement of a contact lens's oxygen transmissibility.

Drainage angle

In glaucoma terminology, "angle" refers to the drainage channel for the aqueous humor in the eye; improper drainage can lead to the high intraocular pressure associated with glaucoma. In narrow-angle glaucoma, the channel is blocked, whereas open-angle glaucoma has other causes, such as the body producing too much aqueous humor.

Drooping eyelids

Drooping eyelids, also called ptosis, is a condition in which the upper eyelid(s) sag. It can be present at birth or caused later in life due to a problem with the muscles lifting the eyelid, called levators.

Drugs

Many drugs, both legal and illegal, can affect your eyes and vision. These include eye drops, other topical eye medications, pills and more. Signs and symptoms can include blurred vision, burning, dry eyes, eyelash loss, floaters, halos around lights, light sensitivity, pupils that are dilated, small or unresponsive to light, peripheral or general vision loss, and jaundice.

Druse

A druse is a single small, yellow or white deposit in the eye. Drusen is the plural form of the word. Drusen are sometimes signs of age-related macular degeneration (AMD), but can be found in the peripheral retina, too.

Dry eye 

Dry eye refers to a lack of sufficient lubrication and moisture in the eye. Many dry eye complaints are temporary and easily relieved.

Dry eye syndrome (DES)

Dry eye syndrome, also called dry eye disease (DED) or keratoconjunctivitis sicca (KCS), is chronic dryness due to a reduced quality or quantity of the eye's tear film, or due to increased evaporation of the existing tear film. Dry eye syndrome has many causes, including aging, certain systemic diseases and long-term contact lens wear. Additional signs and symptoms can include foreign body sensation, eye pain or discomfort, burning, grittiness, itching, light sensitivity, frequent blinking, a red or pink eye, and tearing. Dry eye syndrome is chronic and typically needs treatment by an eye care practitioner.

Dry skin

Skin that is generally dry may include dry eyelids.

E-H

Ectropion

Ectropion is the abnormal turning out of an eyelid, typically the lower one, which exposes the inner, conjunctival side of the eyelid. It often occurs due to aging. Additional signs and symptoms can include eye or lid pain or discomfort, a red or pink eye or eyelid, and overflow tearing.

Edema

Edema is the accumulation of an excessive amount of watery fluid, which causes swelling.

Electroretinogram (ERG)

An electroretinogram is a diagnostic test that measures the electrical responses of the retina’s photoreceptors and other cells in response to flashes of light. It is used to evaluate inherited retinal diseases such as retinitis pigmentosa, as well as other conditions that affect retinal function.

Emmetropia

Emmetropia is a refractive condition where a person can see distant objects clearly with the eyes "at rest" (no focusing effort required) and needs to exert only the normal amount of focusing effort (determined by the distance of an object from the eye) to see near objects clearly. An emmetropic eye has no nearsightedness, farsightedness or astigmatism.

Endophthalmitis

Endophthalmitis is inflammation of the interior of the eye, typically caused by an infection from eye surgery or trauma. Endophthalmitis is an ocular emergency. Signs and symptoms can include floaters, light sensitivity, eye pain or discomfort, a red or pink eye, and vision loss.

Enophthalmos

Enophthalmos is the sinking of the eye into the socket. Causes can include development problems in utero, trauma and inflammation.

Endpiece

Endpieces are located on the left and right of the front of an eyeglass frame and connect the frame to the temples (arms). They’re typically, but not always, connected with a screw-mounted hinge.

Entropion

Entropion is the abnormal turning in of an eyelid, which causes the lashes to rub on the ocular surface. It often occurs due to aging. Signs and symptoms can include eye or eyelid pain or discomfort, a foreign body sensation, a red or pink eye, itching, tearing and vision loss.

Environmental condition

Environmental factors can affect your health and comfort, including that of your eyes. For instance, air pollution, wind and bright light can irritate your eyes and cause signs and symptoms such as burning, dryness and tearing.

Enzymatic cleaner

An enzymatic cleaner removes protein deposits and other debris from contact lenses. It's typically recommended for use either daily, weekly or monthly. Some enzymatic cleaners are a small tablet dropped into a solution along with the lens; others come in liquid form.

Epiphora

Epiphora is the medical term for excessive tearing or overflow of tears down the cheek. It can be caused by overproduction of tears (in response to irritation, allergy or dry eye) or by blocked or inefficient tear drainage through the puncta and nasolacrimal duct.

Epiretinal membrane (ERM)

An epiretinal membrane, also called a macular pucker or cellophane maculopathy, is a thin layer of scar tissue on the retina. Epiretinal membranes have a variety of causes, including vitreous detachment, but the cause is often unknown. In its early stages, an epiretinal membrane is commonly asymptomatic, but some people may have blurred vision or metamorphopsia.

Episclera

This is the outer layer of the eye's sclera (the white part of the eye) that loosely connects it to the conjunctiva.

Episcleritis

Episcleritis is inflammation of the episclera. The cause is usually unknown, but it may be associated with some systemic (e.g., autoimmune) diseases. Signs and symptoms can include a red or pink eye, eye pain or discomfort, light sensitivity and tearing.

Epithelial ingrowth

Epithelial ingrowth is a LASIK complication in which epithelial cells grow under the LASIK flap. It is often temporary and doesn't usually affect vision. In some cases, however, it can affect vision and requires the flap to be lifted and the cells removed.

Esotropia

Esotropia is a type of strabismus that occurs when one or both eyes point inward, so the eyes are "crossed."

Excimer laser

This is an instrument that uses shorter wave (ultraviolet) light to vaporize and remove tissue from the eye's surface during vision correction procedures, such as LASIK and PRK.

Exophthalmos (proptosis)

Exophthalmos, also called proptosis, is the forward protrusion or bulging of one or both eyes from the eye socket. Causes can include thyroid eye disease (the most common cause in adults), orbital cellulitis, orbital tumors, vascular abnormalities and trauma. Signs and symptoms can include a noticeably bulging eye, eyelid retraction, dryness or irritation from incomplete eyelid closure, double vision and, in severe cases, vision loss from optic nerve compression. Prompt evaluation is important to identify and treat the underlying cause.

Exotropia

Exotropia is a type of strabismus that occurs when one or both eyes point outward.

Extended wear

Extended wear contact lenses are those that can be worn continuously, sometimes during sleep. Some brands are FDA-approved to be worn without removal for up to seven days.

Eye care practitioner

Eye care practitioners include optometrists, ophthalmologists and opticians, whose training and roles overlap in some areas and differ in others. Optometrists (ODs) complete four years of optometry school after college and provide primary eye care — comprehensive eye exams, prescribing glasses and contact lenses, and diagnosing and treating many eye conditions. In some states, optometrists are authorized to perform certain in-office laser procedures and minor surgeries; their scope of practice varies by state. Ophthalmologists (MDs or DOs) are physicians who complete medical school followed by an ophthalmology residency, and many pursue additional subspecialty fellowships. They provide both medical and surgical eye care, including major surgeries. Opticians are trained to fit and dispense eyeglasses and sunglasses (and, in some states, contact lenses) based on a prescription from an OD or MD. They also help patients select frames and lenses, and may adjust or repair eyewear. Their scope of practice and licensing requirements can vary by state.

Eye herpes

See ocular herpes.

Eye tumor

An eye tumor is a growth or mass that occurs in or next to the eye. Specific tumors, both benign and malignant, include the basal cell carcinoma (BCC), capillary hemangioma, cavernous hemangioma, choroidal hemangioma, choroidal melanoma, conjunctival intraepithelial neoplasia (CIN), conjunctival lymphoma,  conjunctival melanoma (Co-M), conjunctival squamous cell carcinoma, dermoid cyst, iris melanoma, lacrimal gland carcinoma, optic nerve glioma, optic nerve meningioma, orbital lymphoma, ocular surface squamous neoplasia (OSSN), primary vitreoretinal lymphoma (PVRL), retinoblastoma, retinal hemangioma, rhabdomyosarcoma, sebaceous gland carcinoma, squamous cell carcinoma (SCC) and uveal melanoma (UM). The cause is dependent on the type of tumor present. Signs and symptoms can include blurred vision, a bulging eye, double vision, floaters, a foreign body sensation, pain or discomfort in the eye, the eyelid or around the eye, swelling of the lid or around the eye, a red or pink eye, ptosis, vision loss, limited eye or lid movement, a white or cloudy spot on the eye or an iris defect.

Farsightedness

See hyperopia.

Food & Drug Administration (FDA)

The Food & Drug Administration is a U.S. government body that oversees medical devices and medications, including contact lenses, intraocular lenses, excimer lasers and eye drops. In the United States, these products must be approved by the FDA before they can be marketed.

Femtosecond laser

A femtosecond laser is a device that creates bursts of laser energy at an extremely fast rate measured in terms of a unit known as a femtosecond (one quadrillionth of a second). These ultra-fast energy pulses precisely target and break apart tissue or other substances at a molecular level, without damaging adjacent areas.

Fibromyalgia

Fibromyalgia is a condition that can cause long-term pain all over the body and at tender points in soft tissues, such as muscles, tendons and joints. The cause is unknown, but proposed triggers include trauma, sleep problems, an infectious microbe, depression, chronic back pain and hypothyroidism.

Fixation

In terms of vision, fixation refers to the eye's ability to maintain a gaze upon an object.

Floaters

Floaters are small shapes — spots, specks, strands, cobwebs or squiggles — that drift across your field of vision and move as your eye moves. Most floaters come from age-related changes in the vitreous, the gel-like substance that fills the back of the eye. As the vitreous liquefies, tiny clumps of collagen cast shadows on the retina that you see as floaters. Occasional floaters are common and usually harmless. A sudden increase in floaters, a “shower” of new floaters, flashes of light, or a curtain or shadow moving across your vision can be signs of a retinal tear or detachment, which is a medical emergency. See an eye doctor promptly if these symptoms occur.

Fluorescein

Fluorescein is an orange dye used in eye care that glows bright yellow-green when illuminated with blue (cobalt) light. Eye doctors place fluorescein on the surface of the eye to highlight corneal abrasions, ulcers, dry spots and tear film irregularities. It can also be injected intravenously during fluorescein angiography to image the blood vessels of the retina.

Fluorescein angiography

Fluorescein angiography is an imaging test that involves first injecting fluorescent yellow-green dye into the veins. When the dye reaches the interior regions of the eye, it provides an opportunity for high contrast photography or other imaging of blood vessels. Fluorescein angiography can be particularly useful in diagnosing conditions such as age-related macular degeneration, which in advanced forms, can be characterized by abnormal growth of blood vessels in the retina.

Forceps

Forceps are a surgical tool shaped like tongs, used for gripping.

Foreign body

A foreign body is something in or on the eye that doesn't belong there. Signs and symptoms can include a foreign body sensation, eye pain or discomfort, a red or pink eye, tearing, frequent blinking, blurred vision, discharge, light sensitivity and vision loss.

Foreign body sensation

A foreign body sensation is the feeling that something is in your eye.

Fovea

The fovea is a depression in the retina that contains only cones (not rods), and that provides acute eyesight.

Free radicals

Free radicals are atoms or molecules with unpaired electrons that are highly chemically reactive. Free radicals are capable of causing tissue damage and accelerating the effects of aging through a process called oxidation.

Frequent replacement contact lenses

Frequent replacement contact lenses, called planned replacement contact lenses, are any contact lens that is thrown away after a moderately short period of time. Among many eye care practitioners, "disposable" usage ranges from one day to two weeks, while "frequent replacement" lenses are discarded monthly or quarterly.

Fuchs’ endothelial dystrophy

Fuchs’ endothelial dystrophy is an inherited condition in which the endothelial cells lining the inside of the cornea gradually die off. As fewer endothelial cells remain to pump fluid out of the cornea, the cornea swells and becomes cloudy. Symptoms include morning vision blurring that improves through the day, glare and progressive vision loss. Advanced cases are treated with partial-thickness corneal transplant (DMEK or DSAEK).

Fundus

The fundus is the inside back surface of the eye, including the retina, macula, optic disc and retinal blood vessels. It is the part of the eye examined during a dilated eye exam or with fundus photography.

Fungal keratitis

Fungal keratitis is a serious eye infection typically caused by fungi found in places such as soil, water and organic matter, including plants. Three types of fungi commonly associated with this infection include Fusarium, Aspergillus and Candida. Ordinarily, it is rare for fungi to invade and damage the eye. But symptoms of fungal keratitis can be severe, and the condition requires immediate treatment.

Geographic atrophy

Geographic atrophy (GA) is an advanced form of dry age-related macular degeneration in which patches of cells in the macula — including the retinal pigment epithelium and overlying photoreceptors — progressively die off. These patches expand over time and cause permanent loss of central vision in the affected areas. GA does not always involve abnormal blood vessel growth (unlike wet AMD). Intravitreal eye injections, including pegcetacoplan and avacincaptad pegol, are FDA-approved to slow the rate at which GA lesions enlarge; they do not restore lost vision.

Giant papillary conjunctivitis (GPC)

Giant papillary conjunctivitis is an inflammatory condition characterized by the development of large bumps (papillae) on the inner surface of the upper eyelid. It is most commonly associated with contact lens wear but can also occur in response to ocular prostheses, exposed sutures or scleral buckles. Signs and symptoms can include itching, mucous discharge, contact lens intolerance, blurred vision and a sensation that the contact lens is moving excessively. Treatment may include switching contact lens materials or replacement schedules, improving lens hygiene and using prescription anti-inflammatory (corticosteroid) eye drops.

Glaucoma

Glaucoma is a group of eye diseases that damage the optic nerve and can lead to peripheral vision loss and, if untreated, blindness. Elevated intraocular pressure is a major risk factor, but glaucoma can also occur at normal pressures (normal-tension glaucoma). The most common form, primary open-angle glaucoma, develops slowly and usually has no early symptoms, which is why regular comprehensive eye exams are important for early detection. Angle-closure glaucoma can develop suddenly with severe eye pain, redness, blurred vision, halos around lights, nausea and vomiting. It is a medical eye emergency.

Glycemic index

Glycemic index is a method of ranking foods in terms of how quickly they affect blood sugar levels. For example, foods with high glycemic index rankings (processed foods such as white flour, sugar, etc.) can cause unhealthy spikes in blood sugar. Foods with lower glycemic index rankings, such as whole grains, can help create more stable blood sugar levels.

Goldmann applanation tonometry

Goldmann applanation tonometry is the long-standing standard for measuring intraocular pressure. A small, flat-tipped probe is gently touched to the anesthetized cornea, and the pressure needed to flatten a specific area of the cornea is recorded. The technique is commonly used in glaucoma diagnosis and monitoring.

Gonioscopy

Gonioscopy is an exam technique that uses a special mirrored contact lens at the slit lamp to view the drainage angle of the eye — the area where the iris meets the cornea. It is essential in classifying glaucoma as open-angle or angle-closure and in identifying angle abnormalities.

Graves' ophthalmopathy

Graves’ ophthalmopathy is an autoimmune eye disorder usually associated with abnormalities of the thyroid gland. Signs and symptoms can include eyelid retraction, bulging eyes, light sensitivity, eye discomfort, double vision, vision loss, a red or pink eye, and a limited ability to move the eyes.

Hard contact lenses

Rarely worn now, hard contact lenses are the small, hard lenses made of PMMA (poly(methyl methacrylate)) material that many people wore in the '70s and '80s. Compared with modern soft lenses and rigid gas permeable (RGP or GP) lenses, they are less healthy to wear long-term, since the material doesn't allow oxygen to reach the surface of the eye.

Hemianopia (hemianopsia)

Hemianopia, also spelled hemianopsia, is a visual field defect in which a person loses sight in half of the visual field of one or both eyes. Homonymous hemianopia, in which the same side of the visual field is missing in both eyes, is typically caused by damage to the visual pathways behind the optic chiasm, often from stroke, brain tumor or head trauma. Bitemporal hemianopia, in which the outer halves of both visual fields are lost, is most often caused by a lesion at the optic chiasm, such as a pituitary tumor. Visual field testing is used to map the defect, and treatment focuses on the underlying cause.

Hemifacial spasm

A hemifacial spasm is involuntary muscle twitching on one side of the face, typically caused by compression of the seventh (facial) cranial nerve by a neighboring blood vessel somewhere in the brain.

Hemorrhage

The term hemorrhage refers to profuse bleeding.

Herpes of the eye

See ocular herpes.

Heterochromia

Heterochromia is a condition in which one eye is a different color from the other, or one eye is more than one color.

Higher-order aberration

A higher-order aberration is an irregularity of the eye other than a refractive error (myopia, hyperopia or astigmatism). Higher-order aberrations sometimes affect vision (such as decreasing contrast sensitivity), and sometimes do not.

High-index

A high-index lens is a type of eyeglass lens with a higher index of refraction, meaning that light travels faster through the lens to reach the eye than with traditional glass or plastic. It is denser, so the same amount of visual correction occurs with less material (whether glass or plastic), allowing the lens to be thinner.

Histamine

Histamine is a protein that can be released as part of the body's immune system responses during an allergic reaction. The presence of histamine can lead to inflammation and swelling, which is why antihistamines are often prescribed for allergy symptoms.

Histiocytosis

Histiocytosis is the abnormal proliferation of histiocytes (immune system cells). Common signs include bone tumors and skin rashes. If histiocytosis affects the eye, it can cause it to bulge.

Homocysteine

Homocysteine is an amino acid containing sulfur. A high homocysteine level in the blood is a possible risk factor for heart disease. One major study indicates that lowering homocysteine levels through vitamin B supplementation might help prevent diseases associated with impaired function of small blood vessels, such as macular degeneration. Further research is needed to verify this link.

Hordeolum

A hordeolum is the medical term for a stye — a tender, red bump on or in the eyelid caused by a bacterial infection of an eyelid gland. An external hordeolum involves a gland at the base of an eyelash; an internal hordeolum involves a meibomian gland. Warm compresses and gentle lid hygiene are first-line treatment.

Horner's syndrome

Horner’s syndrome is a condition characterized by a small pupil, ptosis and an abnormal lack of facial perspiration (all on the same side of the face). It is typically caused by injury to the sympathetic nerves of the face.

Hybrid lenses

These are lenses that have a central optical zone made of rigid gas permeable (RGP) plastic, surrounded by an outer "skirt" of silicone hydrogel or regular hydrogel material. Hybrid lenses are designed to provide the crisp optics of GP lenses and wearing comfort comparable to that of soft contact lenses.

Hydrogel

Hydrogel is a hydrophilic ("water-loving") plastic material that is rigid when dry but becomes soft and pliable when moist. Hydrogel materials used for soft contact lenses typically range from approximately 38% to 75% water by weight when fully hydrated.

Hyperopia

Hyperopia (farsightedness) is a refractive error in which a person must exert a greater-than-normal focusing effort to see distant and near objects clearly. Depending on the farsighted person's age and degree of hyperopia, objects may be clear or blurred without corrective lenses. Other symptoms of hyperopia can include headaches, eye strain, and avoidance of reading and other near tasks.

Hypoglycemia

Hypoglycemia is an abnormally low level of sugar (glucose) in the blood, which can cause dizziness, hunger, shakiness and other symptoms. Skipping or delaying meals, eating meals that are too small and a high level of physical activity can cause hypoglycemia. Too much alcohol or certain drugs can also lead to the condition. In people with diabetes, it can be caused by too much insulin.

Hyphema

A hyphema is the collection of blood in the anterior chamber of the eye, between the cornea and the iris. It is most often caused by blunt or penetrating trauma to the eye, but it can also result from surgery, abnormal blood vessels associated with eye disease, or bleeding disorders. Signs and symptoms can include visible blood inside the eye, blurred vision, light sensitivity and eye pain. A hyphema requires prompt evaluation because it can lead to elevated intraocular pressure, corneal staining and secondary glaucoma. Treatment may include rest, head elevation, eye shielding, medications to control pressure and inflammation and, in severe cases, surgical intervention.

Hypopyon

A hypopyon is a layer of white blood cells that has settled at the bottom of the anterior chamber of the eye, visible as a pale or yellowish horizontal line. It is a sign of significant intraocular inflammation and can occur with severe infections such as corneal ulcers or endophthalmitis, as well as with certain forms of uveitis (such as Behçet disease and HLA-B27–associated uveitis). A hypopyon typically requires prompt evaluation to identify and treat the underlying cause and help prevent vision loss.

Hypotony

Hypotony is low intraocular pressure, often caused by eye surgery or trauma (e.g., open globe injury). Symptoms can include blurred vision and eye pain or discomfort.

I-L

Idiopathic

The term idiopathic means having an unknown cause. A medical condition that appears suddenly with no apparent explanation is considered idiopathic.

Index of refraction

This is a measure of how much a substance reduces the speed of light waves passing through it. The index of refraction (or refractive index) of a substance equals the ratio of the speed of light in a vacuum to its speed in that substance. Lenses or materials with a high index of refraction slow down and refract (bend) light more than materials with a lower refractive index.

Insulin

Insulin is a hormone that helps regulate blood sugar levels in the body. People with uncontrolled diabetes are at risk of developing diabetes-related eye conditions caused by high blood sugar.

Intermediate vision

Intermediate vision generally refers to eyesight at approximately arm's length, used for tasks such as computer work and viewing the speedometer in a car.

Intraocular lens (IOL)

An intraocular lens is an artificial lens that a cataract surgeon places in a patient's eye after removing the eye's natural crystalline lens. Like a contact lens, it can have a built-in refractive power tailored specifically to the patient's visual condition.

Intraocular lens (IOL) types

Modern intraocular lenses come in several designs. A monofocal IOL provides clear vision at a single distance (typically far). A toric IOL also corrects astigmatism. Multifocal, trifocal and extended depth-of-focus (EDOF) IOLs are designed to provide a range of vision (near, intermediate and distance).

Intraocular pressure (IOP)

Intraocular pressure refers to pressure within the eye, as determined by the amount of aqueous humor filling it. High IOP (ocular hypertension) can be a sign of glaucoma.

Ion

An ion is an electrically charged atom.

Iris

The iris is the pigmented structure surrounding the pupil and determines eye color. The iris also acts as a diaphragm that increases and decreases the size of the pupil to control the amount of light entering the eye.

Iritis

Iritis is inflammation of the iris.

Ischemia

Ischemia refers to poor blood flow. Obstructions such as clots in veins and arteries can block blood flow, depriving the tissues of oxygen and nutrients. These blockages can also cause "eye strokes" and sudden vision loss.

Jaundice

Jaundice is the yellow coloring in the eyes and skin caused by high levels of a pigment called bilirubin. Jaundice is associated with a variety of conditions involving the liver, gallbladder and bile ducts, such as hepatitis and cirrhosis.

Keratectomy

A keratectomy is the surgical removal of part of the cornea.

Keratitis

Keratitis is inflammation of the cornea, caused by an infection or inflammatory process. Signs and symptoms can include eye pain or discomfort, light sensitivity, a foreign body sensation, grittiness and tearing.

Keratoconjunctivitis

Keratoconjunctivitis is inflammation of the cornea and conjunctiva.

Keratoconjunctivitis sicca (KCS)

Keratoconjunctivitis sicca is the chronic lack of sufficient lubrication and moisture in the eye. It is also called dry eye syndrome (DES) or dry eye disease (DED).

Keratoconus

Keratoconus is a progressive condition in which the cornea gradually thins and steepens, bulging forward into a cone-like shape. This irregular curvature distorts how light enters the eye, causing a form of irregular astigmatism. The exact cause isn't fully understood, but risk factors include genetics, chronic eye rubbing, and certain connective tissue and atopic (severe allergy) conditions. Early symptoms often include blurred or distorted vision that doesn't correct well with glasses. Rigid gas permeable or specialty lenses can often provide sharper vision by masking the corneal irregularity, though more advanced cases may require corneal cross-linking to slow progression or, in severe cases, a corneal transplant.

Keratoplasty

Any of several types of corneal surgery, such as shrinking the collagen to reduce farsightedness or transplanting a new cornea to treat keratoconus.

Keratometer

A keratometer is an instrument that measures the curvature of the eye's clear, front surface (cornea). Keratometers help eye doctors collect information for contact lens fittings, corneal diseases, and surgical procedures. With keratometry, reflected images can also help identify dry eyes.

Keratotomy

Keratotomy is a procedure that involves making incisions in the cornea to change its shape to help correct certain refractive errors.

Lacrimal plug

A lacrimal plug, also called a punctal plug, is a device used to block the lacrimal punctum (an opening at the end of a tear duct) to help keep the eye moist.

Lagophthalmos

Lagophthalmos is the inability to fully close the eyelids. Causes can include facial nerve weakness (such as Bell’s palsy), thyroid eye disease, Ramsey Hunt syndrome, Lyme disease, tickborne disease, eyelid scarring or surgery, severe proptosis and certain neurological conditions. Incomplete eyelid closure can lead to corneal exposure, dryness, irritation and, if left untreated, corneal ulceration. Treatment may include lubricating drops and ointments, moisture chamber goggles, eyelid taping at night and, in some cases, surgical procedures to improve closure.

Lambert-Eaton myasthenic syndrome (LEMS)

This is an autoimmune condition that occurs at the junctions of nerve and muscle cells. Disruption of the electrical impulses between these cells can cause signs and symptoms such as muscle weakness, a tingling sensation, dry mouth and fatigue.

Laser-assisted in-situ keratomileusis (LASIK)

LASIK is a refractive surgery in which a thin flap is created in the front of the cornea, an excimer laser reshapes the underlying corneal tissue, and the flap is placed back in position. LASIK can correct nearsightedness, farsightedness and astigmatism, and is sometimes used to create monovision for presbyopia. The flap may be made with a microkeratome (a precision blade) or a femtosecond laser (“all-laser” or “bladeless” LASIK). Related procedures include PRK and SMILE.

Laser-assisted (sub-)epithelial keratomileusis (LASEK)

LASEK is a refractive surgery that is similar to LASIK, except that the eye surgeon cuts a flap into the epithelium only, instead of through the epithelium and part of the stroma. LASEK is used mostly in people with thin or flat corneas who are not candidates for LASIK, which typically requires more corneal tissue for a successful outcome.

Laser peripheral iridotomy (LPI)

Laser peripheral iridotomy is a quick, in-office laser procedure in which a small opening is made in the peripheral iris to allow fluid to flow more freely within the eye. It is used to treat or prevent angle-closure glaucoma in people with narrow drainage angles.

Laser photocoagulation

Laser photocoagulation is a procedure in which an eye surgeon uses a laser to coagulate tissue, usually to seal leaking blood vessels and destroy new ones in diseases like age-related macular degeneration and diabetic retinopathy.

Laser thermal keratoplasty (LTK)

Laser thermal keratoplasty, also called laser thermokeratoplasty, is a surgery performed to correct mild farsightedness in people over 40. The eye surgeon uses a holmium laser to heat the cornea and shrink its collagen.

Lateral rectus muscle

This is a muscle that moves the eye away from the nose.

Lead poisoning

Lead poisoning is a condition resulting from excessive levels of lead in the body. Typically, children get lead poisoning from chewing or consuming lead paint in older homes (lead paint was banned in the 70s). Some people remain symptom-free, but others may suffer convulsions, paralysis, learning difficulties, or abdominal and other pain. Some people can also develop vision loss.

Leber's congenital amaurosis (LCA)

Leber’s congenital amaurosis is an inherited condition characterized by vision loss or blindness at birth or shortly thereafter. The exact cause is unknown, but eye doctors believe that the retina may degenerate or that its photoreceptors may not develop properly. Nystagmus is a common sign.

Legal blindness

Legal blindness is a severe visual disability where the best attainable visual acuity in a person's better eye (with corrective lenses) is 20/200 or worse on a standardized eye chart or a person's peripheral vision is restricted to no greater than 20 degrees at its widest diameter, as determined by visual field testing.

Lens

1. The nearly spherical body in the eye, located behind the cornea, that focuses light rays onto the retina. 

2. A device used to focus light into the eye to magnify or minify images, or otherwise correct visual problems. Eyeglass lenses, contact lenses, and intraocular lenses are examples. Sometimes, a lens provides only a cosmetic benefit, as in non-correcting color contacts, theatrical contact lenses or contacts that hide a disfigurement of the eye. At other times, a lens protects the eye, as in safety glasses or sunglasses. A bandage contact lens may be applied after eye surgery. And contact lenses used in ortho-k or corneal refractive therapy are designed to reshape the cornea for better vision when the lens is removed.

Lens dislocation

Lens dislocation is the full or partial displacement of the eye’s natural crystalline lens or an implanted intraocular lens (IOL). Trauma to the eye or head can dislocate either type of lens. Dislocation of the natural lens may also be inherited or occur as a feature of certain systemic conditions, including Marfan syndrome, Weill-Marchesani syndrome, Ehlers-Danlos syndrome and homocystinuria. Common symptoms include blurred vision and sometimes double vision; lens dislocation can also raise the risk of glaucoma.

Leukocoria

Leukocoria is an abnormal reflection from the pupil that causes it to appear white rather than black or red. It can be a sign of a serious eye condition, such as a congenital cataract, retinoblastoma, intraocular infection, Coat's disease and retinopathy of prematurity.

Limbal relaxing incisions

This is a surgical procedure, often performed during cataract surgery, which can correct mild astigmatism by flattening the curvature of the eye's clear surface (cornea). Limbal relaxing incisions can be performed with a femtosecond laser at the boundary (limbus) separating the cornea from the white of the eye (sclera). By altering this portion of the eye instead of the center, eye surgeons may be able to preserve the surface and optical qualities of the cornea.

Limbus

The limbus is the boundary area connecting the cornea and sclera. These three structures form the eye's outermost layer.

Lipid

A lipid is an organic compound that is oily, fatty or waxy and is commonly found in living cells. Lipids are one component of human tears, forming an oily outer layer that helps keep the eye moisturized by reducing evaporation of the watery and mucus layers beneath it. Lipids can collect on contact lenses, which can make them uncomfortable to wear.

Liver spot

A liver spot is a benign dark spot on the skin, usually brown, that is associated with exposure to sunlight. Liver spots tend to increase in size over time.

Low vision

Low vision is sight that cannot be satisfactorily corrected with glasses, contacts or surgery. Also called partial sight, low vision usually results from an eye disease, such as glaucoma or macular degeneration.

Lutein

Lutein is an antioxidant that is found throughout the body, but it is concentrated in the macula. Lutein is believed to help protect the eyes from free radical damage caused by the sun's harmful rays.

M-O   

Macula

The macula is the most sensitive part of the central retina, responsible for your sharpest visual acuity and color vision.

Macular degeneration

Macular degeneration is any disease that damages the macula, the central part of the retina, leading to loss of detailed central vision. The most common form by far is age-related macular degeneration (AMD). Less common, inherited forms — such as Stargardt disease and Best disease — can affect children, adolescents and younger adults. See age-related macular degeneration for more detail.

Macular edema

Macular edema is swelling of the macula caused by fluid leaking from damaged retinal blood vessels. Common causes include diabetic retinopathy (diabetic macular edema or DME for short), retinal vein occlusion, uveitis and complications after cataract surgery (see Cystoid macular edema). Symptoms can include blurred or distorted central vision. Treatment depends on the underlying cause and may include intravitreal anti-VEGF or steroid injections, focal laser and management of the underlying disease.

Macular hole

A macular hole is an opening in the eye's macula. Many eye doctors believe it can be caused by vitreous shrinkage as we age. Symptoms can include blurring or a blind spot in central vision and metamorphopsia.

Macular telangiectasia type 2 (MacTel)

Macular telangiectasia type 2, often called MacTel, refers to a group of rare retinal diseases in which the small blood vessels around the fovea (the center of the macula) become abnormal. Type 2, the most common form, is a slowly progressive, typically bilateral condition that usually develops in middle age and causes gradual loss of central vision and distortion of straight lines. Diagnosis is made with imaging tests including optical coherence tomography (OCT), OCT angiography and fluorescein angiography. There is an FDA-approved encapsulated cell-based gene therapy that continuously releases ciliary neurotrophic factor (CNTF) from a small intravitreal implant to slow disease progression in MacTel type 2. Low-vision rehabilitation may also be helpful.

Maculopathy

Maculopathy refers to any disease of or damage to the macula, the most sensitive portion of the central retina responsible for detailed vision and color perception. Two examples include age-related macular degeneration and macular edema.

Madarosis

Madarosis refers to eyelash or eyebrow loss. Causes can include infections, metabolic disorders, blepharitis, certain drugs, lupus erythematosus and trauma, among others.

Mast cells

Mast cells are histamine-containing cells that are part of the body's immune system and play a key role in the inflammatory process. Mast cells are located in tissues throughout the body, including the conjunctiva of the eye and eyelids.

Medial rectus muscle

This is the muscle that moves the eye toward the nose.

Median

When applied to numbers, median refers to a value that falls exactly in the middle of a specified range. Half of the numbers are above the median, and half of the numbers are below the median. As an example, "five" is the median of a range of numbers from one to nine.

Meibomian gland

Meibomian glands, found in the eyelid, are glands that produce the oily outer layer of the three-layer tear film that lubricates the eye.

Meibomian gland dysfunction (MGD)

Meibomian gland dysfunction is a chronic condition in which the meibomian glands in the eyelids fail to secrete enough oil or secrete oil of poor quality. Because these oils help prevent tear evaporation, MGD is a leading cause of evaporative dry eye disease. Treatments include warm compresses, lid hygiene, omega-3 supplements, prescription eye drops, and in-office procedures such as thermal pulsation and intense pulsed light (IPL).

Meibomianitis

Meibomianitis is inflammation of the meibomian glands; rosacea is a common cause. Signs and symptoms can include red or pink eyelid margins, a red or pink eye, dryness, burning, blurred vision and eye swelling.

Melanin

Melanin is a pigment that gives color to the iris of the eye as well as other parts of the body, including the skin and hair.

Melanosis

Melanosis is a condition characterized by melanin (pigment) deposits in the skin or eyes.

Meningitis

Meningitis is inflammation of the meninges, membranes that envelope the brain and spinal cord. Viruses and bacteria can cause meningitis. Symptoms can include fever, headache, vomiting, a stiff neck and light sensitivity. It is a serious condition which requires immediate medical attention and typically involves hospitalization.

Meridian

A meridian is one of a number of radially arranged imaginary lines, each of which passes through the center of the pupil when viewing the eye head-on. Generally separated in one-degree increments, meridians are used to determine the location of the most- and least-curved sections of the cornea when prescribing lenses with cylinder power to correct astigmatism. Meridians are also used to describe the shape of corrective lenses.

Metamorphopsia

Metamorphopsia is a vision symptom in which objects appear distorted. For example, straight lines may appear to be wavy, curved or bent, objects may appear to be larger or smaller than they actually are, or closer or farther away than they actually are. Metamorphopsia is typically caused by conditions or diseases that affect the eye's macula and retina.

Microaneurysm

A microaneurysm is a weakened area in the walls of tiny blood vessels. In diabetic retinopathy, microaneurysms can occur in the retina from damage related to abnormally high blood sugar levels. As microaneurysms in tiny blood vessels (capillaries) expand, ruptures can result. These ruptures can lead the hampering of blood flow as well as swelling and leakage that sometimes cause scarring, blind spots and blindness.

Microcornea

The term microcornea refers to an abnormally small cornea.

Microkeratome

A microkeratome is a small blade instrument that eye surgeons use to make incisions in the cornea.

Microphthalmia

Microphthalmia is a congenital defect resulting in an abnormally small eye or eyes. The cause is usually unknown. Microphthalmia typically results in blindness or reduced vision, but normal vision is possible if the eyes are nearly normal in size.

Migraine

A migraine is a severe headache, sometimes accompanied by nausea and visual disturbances. Visual disturbances alone are also possible; this problem is called an ophthalmic migraine, or migraine without headache. Eye and vision symptoms can include blurred vision, ptosis, halos around lights, light flashes, light sensitivity, eye pain or discomfort, vision loss (blind spots in central vision, tunnel vision or overall impaired vision), distorted vision and wavy lines in vision.

Milia

Milia are raised, tiny white bumps on the surface of the skin, often appearing on the eyelid and around the eyes and nose. Milia occur when dead skin cells do not slough off properly and become trapped at the base of a sweat gland or hair follicle, forming a small keratin cyst. They are commonly seen in newborns but can also affect adults of all ages.

Minification

Minification refers to making objects appear smaller; the opposite of magnification. Many lenses for nearsightedness make objects look smaller, and when the lenses are in eyeglasses, they can also make the wearer's eyes look smaller. Aspheric lenses can reduce this minification effect for a more natural look.

Minimally invasive glaucoma surgery (MIGS)

MIGS refers to a group of newer glaucoma procedures that lower intraocular pressure with less tissue disruption and a faster recovery than traditional glaucoma surgeries such as trabeculectomy. Many MIGS procedures are performed at the time of cataract surgery, though some can be performed independently, and most work by enhancing the eye’s natural drainage pathways. MIGS is generally used in mild-to-moderate glaucoma; more advanced disease typically calls for traditional filtration surgery or tube shunt procedures.

Mixed astigmatism

Mixed astigmatism refers to an abnormal curvature of the eye's surface (cornea) that causes focusing problems at both near and distant ranges.

Monochromatic

Monochromatic refers to one wavelength of light, as opposed to the many wavelengths of light found in varying colors.

Monofocal

A monofocal is a type of spectacle lens, intraocular lens (IOL) or contact lens design that has only one corrective lens power. In contrast, a multifocal lens has more than one lens power, enabling sight at multiple distances (focal points).

Monovision

Monovision is a vision correction method for those with presbyopia, in which one eye is corrected for near vision and the other for distance vision, either through contact lenses or refractive surgery. Monovision can reduce or eliminate the need for reading glasses, but it does have some drawbacks, including decreased depth perception.

Mucin

Mucin is a lubricant, such as saliva, that helps protect certain surfaces in the body. In the eyes, mucin is a tear layer that helps moisten and protect the eye's surface.

Mucormycosis

Mucormycosis is a fungal infection typically occurring in the sinuses or lungs. It mainly affects people with compromised immune systems and those who have uncontrolled diabetes. Signs and symptoms can include sinusitis, eye and facial pain, fever, a bulging eye and vision loss.

Multifocal

This is a type of spectacle lens, intraocular lens (IOL) or contact lens design that includes more than one area through which the eye focuses, such as trifocal or progressive lenses. This enables sight at multiple distances, typically for people with presbyopia.

Myasthenia gravis (generalized and ocular)

Myasthenia gravis is a condition causing weakness of the voluntary muscles and is believed to be autoimmune in nature. It is typically categorized as either generalized myasthenia gravis (GMG) or ocular myasthenia gravis (OMG). OMG affects the eye and eyelid muscles, and can include signs and symptoms such as double vision and eyelid ptosis. GMG also affects muscles throughout the body, such as the arms, legs and muscles involved in speaking, swallowing and (in severe cases) breathing.

Mydriasis

Mydriasis is the dilation of the pupil. It can occur naturally in low light or in response to emotion, as a side effect of medications, after the use of dilating eye drops, or as a sign of certain neurological conditions.

Myelin

Myelin is a sheath made of proteins that covers nerve fibers. It is essential to the transmission of nerve impulses carrying information to and from various parts of the body. When myelin is destroyed or damaged in the optic nerve, it can result in optic neuritis, which can cause vision distortion or vision loss.

Myokymia

Myokymia refers to a common eyelid twitch typically brought on by stress or fatigue.

Myopia

Myopia, also called nearsightedness, is a refractive error in which distant objects appear blurred while near objects can be seen clearly. It typically begins in childhood and may progress through the school years. The primary symptom is squinting to see at a distance. Myopia is corrected with glasses, contact lenses or refractive surgery. In children, several approaches — including certain spectacle lens designs, soft multifocal or dual-focus contact lenses, orthokeratology and low-dose atropine eye drops (off-label, not FDA-approved) — can help slow how quickly myopia progresses. Higher levels of myopia carry an increased lifetime risk of retinal complications, including retinal detachment and myopic maculopathy.

Nanometer

A nanometer (nm) is a measurement of length equal to one-billionth of a meter. Wavelengths of light are measured in nanometers. Eyeglass and sunglass lens manufacturers use nanometers to describe the different types of light that may pass through or be blocked by a lens, whether it be visible light, ultraviolet light, blue light, etc. For example, visible light has a wavelength range of 400 to 700 nm. The amount of light transmission itself is described as a percentage. For example, a dark sunglass lens might allow only 12% of visible light to pass through to the eye.

Nearsightedness

See myopia.

Near vision

Near vision refers to eyesight used for reading and other close-up tasks, generally at a range of 12 to 16 inches from the eyes.

Neovascularization

Neovascularization is the abnormal growth of new blood vessels, such as in an excessive amount, in tissue that normally does not contain them or of a different kind than is usual in that tissue.

Neuronal ceroid lipofuscinosis

This is a rare, hereditary, degenerative disease in which the body does not store pigments called lipofuscins properly. It is characterized by vision loss, seizures and dementia. The various types include Santavuori-Haltia (infantile), Jansky-Bielschowsky (late infantile), Spielmeyer-Vogt (juvenile) and Kufs' disease (adult). Some types may also be called Batten disease.

Neurons

Neurons are nerve cells that form intricate networks through which sensory information is transmitted via electrical impulses sent throughout the body. Neurons are part of the central and peripheral nervous systems, which support functions such as thought, sight, perception, hearing, taste, feeling, speech and movement.

Neuroretinitis

Neuroretinitis is inflammation of the optic nerve and retina, commonly caused by an infection. Symptoms can include blurred vision, headache, floaters, eye pain or discomfort, vision loss and loss of color vision.

Neurotrophic keratitis (NK)

Neurotrophic keratitis is a rare corneal disease in which damage to the nerve that supplies sensation to the cornea causes the corneal surface to break down. Because the eye loses its normal protective reflexes and ability to heal, small surface problems can progress to non-healing sores or ulcers. Reduced sensation also means the eye may not feel as painful as it looks, so the condition is often diagnosed late. Common causes include prior herpes eye infections, diabetes, long-term contact lens wear, and surgery or injury affecting the nerves of the face. Treatment ranges from lubrication and protective measures in mild cases to specialty contact lenses, amniotic membrane or prescription nerve growth factor eye drops in more advanced disease.

Nevus

A nevus is a birthmark, freckle or mole that is often brownish, but can be other colors as well. It can occur on the skin or inside the eye and can sometimes become a melanoma, a type of cancerous growth. If your eye doctor discovers a nevus within your eye, they may want to check it regularly to see if it grows or becomes a condition that requires treatment.

Nickel

Nickel is a metallic element used mainly in alloys. Many eyeglass frames are made of nickel alloys. People who are allergic to nickel will likely need to choose a hypoallergenic substitute, such as titanium.

Night blindness (nyctalopia)

Night blindness, or nyctalopia, is difficulty seeing in low light or at night. It is a symptom rather than a disease, and possible causes include vitamin A deficiency, retinitis pigmentosa, cataracts, uncorrected myopia and certain medications.

Nose pad

Nose pads are small, usually clear pads attached to eyeglass frames that rest on either side of the nose to help support the glasses.

Nystagmus

Nystagmus is a condition characterized by rapid and involuntary eye movement that is oscillating and non-chaotic. Blurred vision may result. Nystagmus typically affects infants and can have a variety of causes.

Ocular herpes

Ocular herpes is a recurrent viral infection that can cause inflammation and scarring of the cornea. It is not sexually transmitted. There are various types of ocular herpes, ranging from herpes keratitis to more serious forms that can lead to blindness.

Ocular hypertension

Ocular hypertension, also referred to as high eye pressure, is a condition in which the intraocular pressure of the eye is elevated above normal, which can increase the risk of glaucoma.

Ocular migraine

An ocular migraine involves visual phenomena that may accompany a migraine headache or that may occur without any headache. They can include light flashes, spots, wavy lines, flickers, zig-zagging lights, semi-circular or crescent-shaped visual defects, and distortions of shapes.

Ocularist

An ocularist is a specially trained professional who designs, fabricates, and fits ocular prostheses (artificial eyes) for people who have lost an eye to disease, trauma, or a congenital condition. The work involves taking an impression of the eye socket, custom-shaping and hand-painting the prosthesis to match the patient's other eye, and providing periodic care such as polishing and refitting. In the United States, ocularists are certified by the National Examining Board of Ocularists (NEBO).

OD

This is the abbreviation for "oculus dexter," the Latin term for "right eye." It can also refer to Doctor of Optometry.

Onchocerciasis

Commonly called "river blindness," onchocerciasis is caused by a parasitic worm, which is spread in the human bloodstream through bites from blackflies and buffalo gnats found in parts of Africa, South America and Central America. The worm's offspring can cause inflammation, bleeding and other problems in the eye.

Ophthalmologist

An ophthalmologist is a physician — a medical doctor (MD) or doctor of osteopathic medicine (DO) — who specializes in the medical and surgical care of the eye. Training includes medical school followed by an ophthalmology residency, and many ophthalmologists complete additional subspecialty fellowships in areas such as retina, cornea, glaucoma, pediatrics, oculoplastics, or neuro-ophthalmology. Ophthalmologists perform comprehensive eye exams, diagnose and treat eye disease, prescribe medications, and perform eye surgery. They may also write prescriptions for eyeglasses and contact lenses.

Ophthalmoplegia

Ophthalmoplegia refers to eye muscle paralysis. Causes can include stroke, multiple sclerosis, tumors, thyroid disease, migraines and progressive supranuclear palsies. Signs and symptoms can include limited eye movement, blurred vision, double vision, nystagmus and ptosis. Tolosa-Hunt Syndrome, often called "painful ophthalmoplegia," is characterized by intense pain behind the eye and a headache.

Opsoclonus

Opsoclonus refers to rapid and involuntary eye movement that is irregular and chaotic; it is sometimes called "dancing eyes." It is typically a characteristic of Opsoclonus-myoclonus syndrome.

Optic

Optic refers to the rounded, central portion of an intraocular lens (IOL) used in cataract surgery. The term also generally refers to the eyes or vision.

Optical coherence tomography (OCT)

Optical coherence tomography is a method of imaging that uses light waves to provide cross-sectional views of the interior eye structures.

Optical coherence tomography angiography (OCTA)

OCT angiography is a non-invasive imaging technique that uses optical coherence tomography to visualize the blood vessels of the retina and choroid without the use of injected dye. It is used in the evaluation of diabetic retinopathy, age-related macular degeneration, retinal vein occlusion and other vascular eye conditions.

Optician

In the United States, opticians are eye care professionals who are trained and licensed to sell and fit eyeglasses, sunglasses and specialty eyewear that are made to an optometrist's or ophthalmologist's prescription. Many also have equipment on the premises so they can grind lenses and put them in frames without ordering from a lab. In some states, opticians can, after special training, become certified to fit contact lenses. (Please visit the Opticians Association of America website for licensing requirements for various states.

Optic nerve

The optic nerve carries electrical impulses from photoreceptor cells (rods and cones) in the retina of the eye to the visual cortex in the brain.

Optic nerve cupping

Optic nerve cupping refers to the appearance of an enlarged central depression in the optic disc when retinal nerve fibers are lost. Increasing cupping over time is a hallmark sign of glaucoma damage and is monitored with disc photography, OCT and visual field testing.

Optic nerve head

The optic nerve head, also called the optic disc, is a circular area where the optic nerve enters the retina. It is also the location of the eye's blind spot.

Optic nerve problem

The optic nerve (second cranial nerve) is the part of the eye that carries stimuli from the rods and cones to the brain. Problems including inflammation (optic neuritis), tumors and swelling can lead to symptoms such as blurred vision, loss of color vision, floaters, headache, eye pain or discomfort, nausea and vision loss.

Optic neuritis

Optic neuritis is inflammation of the optic nerve, often resulting in pain with eye movement along with partial visual impairment too complete vision loss in the affected eye(s). It is often associated with multiple sclerosis (MS) but can also occur with neuromyelitis optica spectrum disorder (NMOSD), myelin oligodendrocyte glycoprotein antibody disease (MOGAD), infections and other inflammatory conditions. Additional signs and symptoms can include reduced color vision (especially desaturation of red), a relative afferent pupillary defect and a central blind spot. Treatment depends on the cause and may include corticosteroids and management of any underlying systemic condition.

Optic papilla

The optic papilla is where the optic nerve exits the eye. It is also referred to as the optic disc or optic nerve head.

Optometrist

An optometrist, or Doctor of Optometry (OD), is a primary eye care provider trained to perform comprehensive eye exams, prescribe eyeglasses and contact lenses, and diagnose and manage many eye and vision problems. Optometrists may also prescribe ophthalmic medications, frequently comanage pre- and postoperative care for eye surgeries, and, in some states, perform certain in-office laser procedures and minor surgeries (scope of practice varies by state). Becoming an optometrist requires four years of optometry school after college, and some optometrists complete additional residency training in areas such as ocular disease, primary care, or pediatrics.

Orbit

The term orbit refers to the eye socket. Read more about the orbital bones.

Orbital cellulitis

Orbital cellulitis is a sudden infection of the tissues immediately surrounding the eye, often resulting in painful swelling of the upper and lower eyelid and possibly the eyebrow and cheek. Other signs and symptoms can include bulging eyes, decreased vision, fever and eye pain when moving the eyes. Bacteria from a sinus infection are a common cause; other causes include a stye on the eyelid, bug bites or a recent eyelid injury. Orbital cellulitis is a medical emergency, and prompt IV antibiotic treatment is often needed to reduce the risk of optic nerve damage, permanent vision loss or blindness, and other serious complications.

Orbital pseudotumor

An orbital pseudotumor is an inflammatory mass in the tissues around or behind the eye that looks like and mimics the symptoms of a tumor. The cause is typically unknown. The primary signs and symptoms are often a painful, bulging eye. Pain or discomfort around the eye may also occur.

Orphan drug

An orphan drug is one designated for the treatment of a rare condition or disease that typically affects fewer than 200,000 U.S. residents. When the FDA grants orphan drug status, a manufacturing company may be qualified for special grants, tax breaks or other incentives to help defray research and production costs.

Orthokeratology (Ortho-k)

Orthokeratology is a procedure in which special gas permeable (GP) lenses are used to reshape the cornea and correct errors like nearsightedness. Often, patients only wear the lenses at night.

Orthoptist 

An orthoptist is an allied eye care professional who specializes in evaluating and non-surgically managing disorders of eye movement, eye alignment, and binocular vision. Orthoptists commonly help manage conditions such as strabismus, amblyopia, convergence insufficiency, diplopia (double vision), and nystagmus, often in pediatric or neuro-ophthalmology settings, and work alongside ophthalmologists, optometrists and opticians as part of the eye care team. In the United States, orthoptists complete a 24-month fellowship program accredited by the American Orthoptic Council (AOC) and earn the Certified Orthoptist (CO) designation after passing board examinations.

OS

This is the abbreviation for "oculus sinister," the Latin term for "left eye."

Osteopetrosis

Osteopetrosis is a rare, hereditary disease in which the bones are too dense. Common signs and symptoms can include bone pain and fractures. The condition can also lead to retinal degeneration, which could result in vision loss.

OU

This is the abbreviation for "oculus uterque," the Latin term for "each eye." It is used in vision correction prescriptions to indicate both eyes. It is also an abbreviation for "oculi unitas" or "oculi uniter," meaning both eyes working together.

Overconvergence

Overconvergence is a condition in which the eyes move too far inward when focusing on a near object, resulting in blurry vision.

Overflow tearing

Overflow tearing is a common congenital condition in infants and a developmental condition in older adults caused by a blocked tear duct. In infants, a membrane blocks the tear drainage duct, resulting in excessive tears and mucus. In adults, the cause of the blockage is usually unknown, but it can be related to poor eyelid function.

P

Pachymetry

Pachymetry is the measurement of corneal thickness, performed with a device called a pachymeter. Corneal thickness affects how intraocular pressure readings are interpreted and is an important factor in evaluating candidates for corneal refractive surgery, monitoring corneal dystrophies and ectasias, and assessing glaucoma risk. Common pachymetry methods include ultrasound and optical coherence tomography (OCT). 

Pan-retinal photocoagulation (PRP)

Pan-retinal photocoagulation is a laser procedure used to treat proliferative diabetic retinopathy and other conditions that cause abnormal new blood vessels in the retina. Small laser burns are placed across the peripheral retina to reduce its oxygen demand, which causes the abnormal new vessels to regress.

Papilledema

Papilledema refers to swelling with accompanying compression of the optic nerve head, which can be a medical emergency. Causes of papilledema can include bleeding near the vicinity of the optic nerve and abnormally high cerebrospinal fluid (CSF) pressure. Autoimmune disorders, trauma and infections of the central nervous system can also cause papilledema. Optic nerve swelling can also result from central retinal vein occlusion, or it may occur as a side effect of medications, such as lithium, tetracycline or corticosteroids.

Papilloma

A papilloma is usually a benign tumor, such as a wart or a skin tag. Papillomas may be raised or flat, and can be a variety of colors, such as skin-colored, yellow, pink, brown or black. Eye papillomas are typically on the eyelid but may also appear on the conjunctiva. The cause of papillomas is thought to be viral.

Parinaud syndrome (dorsal midbrain syndrome)

Parinaud syndrome is a rare disorder that impairs the ability to look upward. It is typically associated with a brain lesion, and it is characterized by nystagmus and pupil unresponsiveness to light. Causes can include hydrocephalus ("water on the brain") and tumors of the pineal gland.

Parkinson's disease

Parkinson’s disease is a neurological disorder characterized by tremors, muscle rigidity, a shuffling walk and a mask-like appearance in the face. It may also cause infrequent blinking.

Pars plana

The pars plana is the posterior part of the eye's ciliary body.

Patau syndrome

Patau syndrome, also called Trisomy-13, is a condition caused by an extra, third copy of chromosome 13. Signs and symptoms can include severe intellectual disability, a small head, microphthalmia, a cleft lip or palate, heart defects and extra fingers or toes. Many patients also have an iris coloboma and retinal dysplasia (abnormal development).

Pediculosis

Pediculosis is a parasitic (lice) infestation, typically caused by contact with an infected person or infected bedding. When lice infest the eyelid and eyelashes, they can cause signs and symptoms such as visible lice (white or gray), eggs called nits (white or gray) or feces (reddish-brown), blue bite marks, blepharitis and conjunctivitis. Some people also develop keratitis.

Penetrating keratoplasty

Penetrating keratoplasty is a transplant procedure in which a circular area of surface eye tissue is removed from a healthy donor cornea and transferred to a recipient. A penetrating keratoplasty or corneal transplant may be needed in case of eye damage from injury or from eye diseases such as keratoconus.

Periorbital cellulitis

Periorbital cellulitis, also called preseptal cellulitis, is an infection and inflammation of the eyelid and portions of skin around the eye, caused by bacteria, viruses or other pathogens.

Peripheral vision

Peripheral vision refers to your side vision or the edges of your visual field. It is sometimes referred to as what you see “out of the corners of your eyes.”

Phacoemulsification

Phacoemulsification, also called "phako," is an in-office cataract surgery procedure. It involves using a device with a vibrating, ultrasonic tip to break up the cataract, then suctioning out the pieces before inserting an artificial intraocular lens.

Phakic

Phakic refers to an eye that still has its natural crystalline lens. An eye is aphakic if the natural lens has been removed (for example, during cataract surgery) and not replaced.

Phakic IOL (pIOL)

A phakic intraocular lens is a lens implanted in the eye in addition to (rather than replacing) the natural lens, to correct moderate-to-high refractive errors such as severe myopia. Phakic IOLs are an option for patients who are not good candidates for LASIK or PRK.

Phoropter

A phoropter is a device that provides various combinations of lenses used for testing vision errors in eye examinations.

Photoablation

Photoablation is a procedure in which a surgeon uses ultraviolet radiation to remove tissue.

Photochromic

Photochromic refers to the ability of a lens to change color or darkness/density depending upon the degree of exposure to light.

Photocoagulation

Photocoagulation is a procedure that uses heat from a high-energy laser to seal off bleeding in damaged tissue. Photocoagulation also may help prevent the formation of abnormal blood vessels (neovascularization) in eye diseases, such as age-related macular degeneration or diabetic retinopathy. Photocoagulation also may be used to reattach a detached retina.

Photokeratitis

Photokeratitis is a condition caused by exposure to ultraviolet (UV) light. It is sometimes referred to as a "sunburn" of the cornea. Symptoms can include eye discomfort, blurry vision and light sensitivity. When the condition is caused by UV rays reflecting off of snow or ice, it is referred to as "snow blindness."

Photophobia

Photophobia (light sensitivity) is eye discomfort from the sun or other light sources. Photophobia can have many causes.

Photopsia

Photopsia refers to seeing flashes of light in the edges of your visual field. It can have many causes, including mechanical (rather than visual) stimulation of light-sensitive cells (photoreceptors) in the retina. For example, a detached retina can cause photopsia when the retina pulls away or detaches from tissue in the back of the eye. A vitreous detachment with accompanying photopsia can occur when the eye's gel-like interior begins to shrink and pull away from the retina. Photopsia can be accompanied by a shower of spots and floaters.

Photoreceptor

A photoreceptor is a light-sensitive cell found in the retina. Photoreceptors in the human retina are classified as cones and rods. Cones are located in the central retina (the fovea) and control color vision. Rods are located outside the fovea and control black/white vision in low-light conditions.

Photorefractive keratectomy (PRK)

PRK is a refractive surgery in which an excimer laser is used to remove corneal tissue to correct vision conditions.

Phytochemicals

These are chemicals found in plants that help protect against disease.

Pigment dispersion syndrome (PDS)

Pigment dispersion syndrome is an eye condition in which pigment granules that normally adhere to the colored part of the eye (iris) flake off and adhere to the posterior surface of the cornea and other structures in the anterior chamber of the eye. If the pigment accumulates in the drainage angle of the anterior chamber, it can reduce the drainage of aqueous from the eye, causing ocular hypertension and (potentially) pigmentary glaucoma.

Pinguecula

A pinguecula is a yellowish, thickened lesion that can develop on the conjunctiva near the cornea. Pingueculae typically represent a benign degenerative change in the conjunctiva caused by the leakage and deposition of certain blood proteins through the permeable capillaries near the limbus.

Placebo

A placebo is a "false" but harmless treatment that has no proven medical value. Placebos are often incorporated into clinical trials to help measure how patients respond to an authentic therapy that is being tested. Some patients are given placebos, and others are given the actual therapy, to provide objective comparisons and assessments.

Plano

Plano is a term eye care professionals use to describe lenses with no corrective power. The term is most often applied to nonprescription sunglasses or contact lenses that are worn for cosmetic purposes only.

Polarized lenses

Polarized lenses contain special filters that block light reflected from horizontal surfaces, such as water, to help reduce glare.

Polycarbonate

Polycarbonate is a lightweight, impact-resistant plastic used in eyeglass lenses, sports eyewear, and safety glasses. It is a common choice for children’s lenses, rimless and drilled frames, and protective eyewear because of its strength. Specialized polyurethane-based lenses are a related material that offers similar impact resistance with slightly better optical clarity and is sometimes preferred for the same applications.

Polymethyl methacrylate (PMMA)

Older versions of hard contacts were made of PMMA, which is not an oxygen-permeable material. Today's rigid gas permeable (RGP) lenses contain other polymers that allow oxygen to reach your eye. Some intraocular lenses are made of PMMA because the material can be well-tolerated inside the eye.

Porphyria

Porphyria is a disorder in which the body produces too much of a compound called porphyrin and releases it in the urine, causing a reddish color. Other symptoms can include light sensitivity, skin that swells or is sensitive to sunlight, abdominal pain, blisters and muscle weakness.

Posterior chamber

This is the part of the eye behind the iris and in front of the lens.

Posterior capsular opacification (PCO)

Posterior capsular opacification, also called a secondary cataract, is a condition that occurs when haziness develops behind the artificial intraocular lens inserted during cataract surgery. A common complication following cataract removal, a PCO can often be removed with a YAG laser capsulotomy.

Posterior synechiae

Posterior synechiae are adhesions that form between the back surface of the iris and the front surface of the lens. They most commonly develop as a complication of inflammation inside the eye, such as uveitis. Posterior synechiae can distort the shape of the pupil and, if extensive, can block the flow of aqueous humor, increasing intraocular pressure and the risk of secondary glaucoma. Treatment typically focuses on controlling the underlying inflammation, dilating the pupil with eye drops to help break or prevent adhesions and managing any pressure rise.

Posterior vitreous detachment (PVD)

Posterior vitreous detachment is a common, usually age-related event in which the gel-like vitreous separates from the back of the eye. Most PVDs are uncomplicated, but the separation can occasionally tear the retina, so a new onset of floaters and flashes warrants prompt evaluation by an eye doctor.

Presbyope

This term refers to a person who has difficulty reading print and seeing near objects due to presbyopia.

Presbyopia

Presbyopia is the gradual, age-related loss of the eye’s ability to focus on near objects. It typically becomes noticeable in the early- to mid-40s, when reading small print or seeing the phone at a comfortable distance becomes difficult. Other symptoms include eye strain, headaches, and the need to hold reading material at arm’s length. Treatment options include reading glasses, bifocal or progressive lenses, multifocal or monovision contact lenses, FDA-approved prescription eye drops, and surgical options such as refractive lens exchange or presbyopia-correcting intraocular lenses.

Prescription lenses

These are lenses that provide vision correction as prescribed by a licensed eye care practitioner.

Prism

In optics, a prism lens is one that can have more precise geometric configurations, enabling light to be bent or reflected in certain ways. A prism can also split white light into different wavelengths and colors.

Procedural pharmaceuticals

Procedural pharmaceuticals are FDA-approved drug-delivery implants placed inside the eye to treat conditions that would otherwise require daily eye drops. Instead of relying on the patient to self-administer medication, these implants release medication steadily over months or years from inside the eye. Some are biodegradable implants placed during a brief in-office procedure, while others are non-degradable implants anchored in the eye's drainage structures during a surgical procedure. The category, sometimes also called sustained-release or intraocular drug delivery, is currently used in glaucoma and is an active area of development across retinal disease and ocular surface disease.

Progressive lenses

Progressive lenses, also called progressive addition lenses (PALs), are multifocal lenses whose corrective powers change progressively throughout the lens. A wearer looks through one portion of the lens for distance vision, another for intermediate vision, and a third portion for reading or close work. Each of the vision zones blends seamlessly into the next, without the noticeable lines seen on traditional bifocal or trifocal lenses.

Proliferative vitreoretinopathy (PVR)

Proliferative vitreoretinopathy is a common complication of surgery for retinal detachment that causes scarring of the retina. It may require a vitrectomy and intricate surgical removal of scar tissue.

Propionate

Propionate is a soft, flexible material that is sometimes used in goggles.

Prosthetic

This term refers to a prosthesis, which is an artificial replacement for a part of the body. In eye care, this may refer to prosthetic contact lenses, used to enhance the appearance of the eyes, or a prosthetic eye placed after the loss of an eye.

Protective eyewear

This typically refers to eyewear made with impact-resistant lenses, usually polycarbonate, that are designed to help protect the eyes, especially in work-related situations or during sports.

Protein

Proteins are large, complex organic molecules found in all living cells. These molecules contain enzymes, antibodies, hormones and other elements that help organisms function. Proteins are present in human tears and can collect on contact lenses, which can lead to discomfort and cloudy vision.

Pseudoexfoliation syndrome (PXF or PEX)

Pseudoexfoliation syndrome is a condition of unknown cause in which light gray, dandruff-like material forms on the pupil margin and anterior lens capsule in the eye. This material can also lodge in the trabecular meshwork, potentially causing an increase in internal eye pressure and pseudoexfoliative open-angle glaucoma.

Pseudotumor cerebri

Pseudotumor cerebri is a condition whose symptoms mirror those of a brain tumor: increased intracranial pressure, headache, nausea, brief periods of vision loss (graying or blurring) and double vision. The cause is unknown, but it often affects women.

Pseudophakic

Pseudophakic describes an eye that has an artificial lens implant following cataract surgery or refractive lens exchange.

Pterygium

A pterygium, also referred to as “surfer’s eye,” is a triangular fold of tissue on the white of the eye that can eventually grow over part of the cornea. The cause may be irritation from the sun’s UV rays, dust or wind. Some people have no signs or symptoms, while others may have redness or blurred vision. Pterygia that are chronically inflamed can cause itching.

Ptosis

Ptosis is the medical term for a drooping eyelid. Congenital ptosis is often caused by a problem with the levator muscle (which lifts the eyelid). In adults, ptosis is commonly caused by the aging of the levator's connective tissue.

Puncta

Puncta are tiny openings through which tears drain away from the eyes. Four puncta are in the nasal corner of the eye — two in the upper inner eyelid and two in the lower inner eyelid. Punctal plugs are sometimes used to block these openings so that more tears are retained as a treatment for dry eye syndrome.

Punctal cautery

Punctal cautery is a procedure that uses heat or laser energy to permanently close channels from which tears drain. It can increase the accumulation of moisture as a treatment for dry eye.

Punctal plugs

Punctal plugs are tiny inserts often made of plastic that are placed in the channels or ducts of the eye where moisture drainage occurs. They can stop excessive drainage to help keep the eye moistened in conditions such as dry eye syndrome.

Pupil

The pupil is the round, dark center of the eye, which opens and closes to regulate the amount of light the retina receives.

Pupillary block

Pupillary block is a mechanism in which the flow of aqueous humor from behind the iris to the front of the eye is obstructed at the pupil. This causes pressure to build up behind the iris, pushing it forward and closing the eye’s drainage angle — the main cause of acute angle-closure glaucoma. It is treated with laser peripheral iridotomy.

Pupillary distance (PD)

Pupillary distance is the distance between the centers of each pupil. Opticians use a special ruler to measure your pupillary distance before ordering your eyeglasses. It is an essential measurement because the optical center of each eyeglass lens must be positioned directly over the center of each pupil. An incorrect measurement means you could have difficulty focusing when wearing the glasses.

Q-S

Radial keratotomy (RK)

Radial keratotomy is a refractive surgery in which incisions are made in the cornea in a radial pattern to flatten the cornea and correct myopia. RK is no longer as widely used as other corrective eye procedures.

Reading glasses

Reading glasses, also called readers, can make it easier to see more clearly up close, particularly for people who are presbyopic.

Refraction

Refraction is a test performed during an eye exam to determine the eyeglass lens powers needed for optimum visual acuity. An automated refraction uses an instrument that does not require the patient to respond. A manifest refraction is the manual way to determine the best lenses, by placing various lenses in front of the patient's eyes and asking, "Which is better, lens A or lens B?"

Refractive error

A refractive error refers to size- and shape-related abnormalities of the eyeball (or components of the eye) that affect the normal ability of the eye to focus light on the retina. Common refractive errors include myopia (nearsightedness), hyperopia (farsightedness), astigmatism and presbyopia.

Refractive surgery

Refractive surgery is performed to correct visual acuity, with the objective of reducing or eliminating the need for glasses and contacts. Examples include radial keratotomy, PRK, LASIK and corneal implants.

Replacement schedule

Replacement schedule refers to how often you discard and replace your contact lenses, which can include every day, week or two weeks (disposable) or every month, two months or calendar quarter (frequent replacement). It's important to differentiate between the replacement schedule and the wear schedule. Wear schedule is either daily wear (removed before sleeping) or extended wear (you may sleep with them in).

Retina

The retina is the sensory membrane that lines the back of the eye. Cells in the retina called photoreceptors transform light energy into electrical signals that are transmitted to the brain by way of the optic nerve.

Retinal artery occlusion

Retinal artery occlusion is a blockage of one of the arteries supplying blood to the retina, often described as a “stroke of the eye.” It causes sudden, painless vision loss and is a medical emergency, both for the eye and because it can signal an underlying stroke risk requiring urgent systemic evaluation.

Retinal detachment (RD)

Retinal detachment is a condition where the retina separates from the choroid. Retinal detachments can have many causes, including aging, surgery, trauma, inflammation, high myopia and diseases such as diabetic retinopathy, retinopathy of prematurity and scleritis. Symptoms can include light flashes, floaters, a shadow coming down over your vision, blurred vision and vision loss.

Retinal pigment epithelium (RPE)

This is the layer of pigmented cells found between the light-sensitive, inner back lining of the eye (retina) and the choroid, which contains blood vessels that supply nutrients and oxygen.

Retinal tear

A retinal tear is a break or split in the retina. It is commonly caused by a vitreous detachment. Symptoms can include floaters and light flashes.

Retinal vein occlusion (RVO)

Retinal vein occlusion is a blockage of one of the veins that drains blood from the retina. It typically causes sudden, painless vision loss in one eye and may lead to retinal swelling, bleeding, and macular edema. Major risk factors include hypertension, diabetes, high cholesterol and glaucoma. Treatment may include anti-VEGF injections, steroid injections, and laser therapy.

Retinitis

Retinitis is inflammation of the retina. Symptoms can include blurred vision, metamorphopsia, floaters and vision loss.

Retinitis pigmentosa (RP)

Retinitis pigmentosa (RP) is a group of inherited retinal disorders that cause progressive damage to the rod and cone photoreceptors. Early symptoms typically include night blindness and gradual loss of peripheral vision (“tunnel vision”), and central vision can be affected in advanced disease. There is no cure for most forms of RP. Genetic testing can identify the specific gene involved, and for one subtype caused by mutations in the RPE65 gene, a gene therapy (voretigene neparvovec) is FDA-approved. Other genetic subtypes are the focus of active research.

Retinopathy of prematurity (ROP)

Retinopathy of prematurity is an eye disease common in premature babies that involves the abnormal growth of blood vessels in the inner back of the eye (retina).

Retinoschisis

Retinoschisis is a condition in which the retina splits into layers, sometimes causing blurred vision. It can be inherited or acquired; the acquired form is typically caused by small cysts in the eye. Floaters may also be a symptom of this condition.

Retrobulbar hemorrhage

Retrobulbar hemorrhage refers to bleeding in the eye socket behind the eye. If left untreated, it can cause increased intraocular pressure (IOP), protrusion of the eyeball and may lead to permanent vision loss.

Rewetting drops

These are eye drops designed to re-moisten and lubricate contact lenses while they are being worn, to increase comfort.

Rhegmatogenous

Rhegmatogenous refers to rising from a break or tear, describing a common type of retinal detachment.

Rheopheresis (RHEO)

The RHEO procedure is a method of blood filtration (apheresis) or treatment of dry age-related macular degeneration that removes large proteins and fatty components from the blood to improve circulation to macular cells at the back of the eye.

Rheumatoid arthritis (RA)

Rheumatoid arthritis is a chronic, autoimmune, inflammatory disorder that mainly affects the joints. Signs and symptoms can include pain, stiffness, swelling and deformities. Rheumatoid arthritis may also cause eye burning, discharge and dryness. It can also be associated with uveitis.

Rigid gas permeable (RGP or GP) lenses

Rigid gas permeable lenses are contact lenses made of breathable plastic that are custom-fit to the shape of the cornea. RGPs are the successor to older hard lenses, which are now virtually obsolete.

Rod

A rod is a photosensitive receptor in the retina that helps you to see in low light.

Rosacea

Rosacea is a skin condition typically involving the face that is often characterized by flushing, red bumps and telangiectasia (dilated, visible capillaries). It is more common among fair-skinned women, who often develop it in their 30s through 50s. When it affects the skin around the eyes, it is referred to as ocular rosacea.

Saccade

A saccade is a rapid eye movement that quickly shifts gaze from one point of fixation to another. Saccades occur, for example, while reading, scanning a scene or moving the eyes between objects. Abnormalities in saccadic movements can be a sign of neurological or ocular motor disorders and are sometimes evaluated as part of a neuro-ophthalmic examination. 

Sarcoidosis

Sarcoidosis is a disease in which granulomas (nodules of inflamed tissue) form in the lymph nodes, lungs, skin and other areas. The cause is typically unknown. Signs and symptoms can include fatigue, weight loss, fever, coughing, uveitis, cranial nerve palsies and skin lesions. Some patients also develop dry eyes.

Schirmer test

The Schirmer test is a clinical test used to measure tear production. A small strip of filter paper is gently placed inside the lower eyelid for several minutes, and the length of the wetted portion is measured. Reduced wetting may suggest decreased tear production (aqueous-deficient dry eye). The test can be performed with or without topical anesthetic and is used as part of the evaluation of dry eye disease, Sjögren’s syndrome and other ocular surface conditions.

Schlemm's canal

Schlemm’s canal is a porous channel within the eye through which fluids drain. When drainage is blocked, high eye pressure can cause eye damage related to glaucoma.

Sclera

The sclera is the outer layer of the eye that forms the visible white of the eye and surrounds the optic nerve at the back of the eye.

Scleral buckle

A scleral buckle is a surgical procedure used to repair certain types of retinal detachment. A small band, typically made of silicone, is sewn onto the outside of the eye (the sclera) to gently indent the eye wall, bringing it closer to the detached retina so that the tear or break can seal. The buckle is usually left in place permanently. Scleral buckling can be performed alone or in combination with other procedures such as vitrectomy or pneumatic retinopexy.

Scleral lens

A scleral lens is a large-diameter rigid gas permeable (RGP or GP) contact lens that vaults over the cornea and rests on the white of the eye (sclera). The space between the lens and the cornea is filled with sterile saline, which provides continuous lubrication of the corneal surface. Scleral lenses are commonly used to provide clearer vision in eyes with irregular corneas (such as in keratoconus, post-LASIK ectasia, or after corneal transplant) and to help support the ocular surface in severe dry eye, neurotrophic keratitis and ocular graft-versus-host disease.

Scleritis

Scleritis is inflammation of the sclera. Autoimmune disorders are the most common cause. Signs and symptoms can include a red or pink eye, eye pain, light sensitivity, tearing and blurred vision.

Scotoma

A scotoma is a blind spot within the visual field.

Seborrheic dermatitis

Seborrheic dermatitis is a skin condition that can cause scaling of the skin, redness and itching. It commonly affects the scalp (dandruff), eyebrows, eyelids, nose, the area behind the ears and the sternum. It can be associated with seborrheic blepharitis.

Segment (Seg)

A segment refers to a part or portion, as in the near-vision portion of a pair of bifocals.

Selective laser trabeculoplasty (SLT)

SLT is an in-office laser procedure used to lower intraocular pressure in open-angle glaucoma. It uses short, low-energy laser pulses to stimulate the eye’s natural drainage system (the trabecular meshwork) to work more efficiently. SLT may be used as a first-line treatment, as an alternative to drops or in addition to medical therapy.

Silicone

Silicone is a type of flexible and comfortable plastic. Because it is commonly used in nose pads in eyeglasses, people who are allergic to silicone should speak with their eye doctor or optician about having glasses made with a different type of nose pad.

Single vision

Single vision refers to a lens that has the same power throughout the entire lens, in contrast to a bifocal or multifocal lens that contains more than one lens power.

Sinusitis

Sinusitis is inflammation of the sinuses, often due to an infection or an allergic reaction. It is a common cause of pain in and around the eye. Signs and symptoms can include head pain (headache, pain around the eyes, toothache or jaw pain), nasal discharge, postnasal drip, coughing, eyelid swelling, swelling around the eyes, a stuffy nose, fatigue, bad breath and a sore throat.

Sjögren's syndrome

Sjögren’s syndrome is an inflammatory autoimmune disorder characterized by a dry mouth and dry eyes. Additional eye signs and symptoms can include burning, discharge, foreign body sensation, itching and light sensitivity.

Skin cancer

Skin cancer is commonly caused by exposure to the sun's UV rays. Types include basal-cell carcinoma, squamous cell carcinoma and melanoma. With carcinoma, you will likely have a red or pink bump that bleeds, crusts and scales. In melanoma, the pigmented areas can be raised or flat; they are often brown or black, but can be (or include) other colors as well, such as blue, red, pink or white. Some raised moles are skin-colored. The more common symptoms of a malignant mole follow an ABCDE checklist:

• Asymmetry – One half does not match the other
• Border – Irregular or scalloped
• Color – Varies in shade or color
• Diameter – Larger than that of a pencil eraser
• Evolving – The mole is changing in size, shape, color, or elevation. It may also start to bleed, itch, or crust over.

Slit lamp

A slit lamp is a specialized microscope that produces a narrow, bright beam of light, allowing an eye doctor to examine the front and middle structures of the eye in three-dimensional detail — including the eyelids, conjunctiva, cornea, anterior chamber, iris and lens. With handheld magnifying lenses, the slit lamp can also be used to examine the retina and optic nerve.

Snellen chart

The Snellen chart is a standard eye chart with letters, numbers, or symbols printed in rows of decreasing size used by eye doctors in distance visual acuity testing. The chart was invented by Dutch ophthalmologist Hermann Snellen.

Soft contact lenses

Soft lenses are made of gel-like plastic containing varying amounts of water.

Solution

A solution, in terms of contact lens care, is a product used to clean, disinfect and store contact lenses.

Spatial frequency

In optics and other fields, spatial frequency refers to a measure of how often a repeating structure (such as a vertical line or bar) appears within a given unit of distance.

Spherical

Spherical refers to a contact lens design that is shaped like a sphere. This type of lens is commonly prescribed to correct refractive errors, such as myopia and hyperopia. In contrast, toric lenses for astigmatism are oval-shaped and are less commonly prescribed.

Spherical aberration

A spherical aberration is a common higher-order aberration of lenses or the eye that often causes glare and halos around lights at night.

Spots

Spots are small, cloudy specks in the eye that become noticeable when they fall in the line of sight.

Sports and performance vision therapy 

Sports and performance vision therapy is a form of vision training designed to enhance the visual skills that contribute to athletic or occupational performance, rather than to treat a clinical disorder. Areas of focus can include dynamic visual acuity, eye tracking, depth perception, peripheral awareness, reaction time and eye-hand coordination. Programs are typically delivered by an eye doctor or vision therapist with additional sports-vision training and may use specialized lenses, strobe glasses, light boards and digital training tools.

Spring hinge

This is a type of hinge on eyeglass frames that is typically more flexible than a regular hinge, which can enhance the durability of the frames.

Stargardt disease

Stargardt disease is an inherited form of macular degeneration that typically begins in childhood or adolescence. It causes progressive loss of central vision due to the accumulation of a fatty substance (lipofuscin) in cells beneath the macula. There is no cure, but low-vision rehabilitation and UV protection are commonly recommended.

Stereopsis

Stereopsis refers to three-dimensional vision, enabling depth perception.

Strabismus

Strabismus is a misalignment of the eyes in which the eyes do not point in the same direction at the same time. Types include esotropia (eye turns inward), exotropia (eye turns outward), hypertropia (eye turns upward), and hypotropia (eye turns downward). The cause varies and may involve the eye muscles, the nerves controlling them, or refractive error; in children, untreated strabismus is a common cause of amblyopia. Strabismus can also affect depth perception. Treatment options include eyeglasses, prism lenses, patching, vision therapy, eye-muscle surgery and, in select cases, botulinum toxin injection. Early evaluation in children is important.

Stroke

A stroke is an interruption in blood flow to the brain, resulting in tissue death. Blood clots and other obstructions can interrupt the blood flow, as can a ruptured artery. Signs and symptoms can include weakness or numbness, particularly on one side of the body, a change in consciousness or mental status, vision loss, double vision and limited eye movement. Frequent blinking may also occur.

Stye

A stye (hordeolum) is a small red bump on the edge of the eyelid caused by an infected gland. Additional signs and symptoms can include eyelid pain, eyelid swelling, eye pain or discomfort, foreign body sensation, light sensitivity and tearing.

Subconjunctival hemorrhage

A subconjunctival hemorrhage refers to bleeding from blood vessels on the surface of the eye that leaves a red patch or spot. This common condition can be caused by sneezing, coughing, high blood pressure and trauma, among other factors.

Sun protection factor (SPF)

SPF is a number that indicates the level of protection against UVB rays a product can offer. In general, the higher the SPF, the greater the level of sun protection.

Superior limbic keratoconjunctivitis

This is a condition characterized by episodes of recurrent inflammation of the superior cornea, the junction between the cornea and the sclera (the limbus), and the conjunctiva lining the superior sclera and inner surface of the upper eyelid.

Surgery complication

Complications from cataract surgery, LASIK or other eye surgeries can result in a variety of signs and symptoms, including blurred vision, ptosis, foreign body sensation, halos around lights, light sensitivity, eye pain or discomfort, red or pink eyes, vision loss and an iris defect.

Suspensory ligament

The suspensory ligament, also called the zonule of Zinn, is a membrane of fibers (zonules) that holds the eye's natural crystalline lens in place.

Systemic lupus erythematosus (SLE)

Lupus, formally called systemic lupus erythematosus (SLE), is an autoimmune inflammatory disease primarily affecting the skin. The exact cause is unknown, but some people appear to have a genetic predisposition to developing lupus, and it is more common in women than men. Systemic symptoms vary depending on the type of lupus present, but red patches on the cheeks are common. When lupus affects the eyes, it can cause signs and symptoms such as a red or pink lid, a spot or scale on the lid that may change in pigment (generally losing pigment except for a darker color marking the border of the spot), eyelash loss, dry eye syndrome, migraine headaches, uveitis, scleritis, conjunctivitis and retinal vascular occlusion (blockage in the retina's vascular system).

T-Z

Tay-Sachs disease

Tay-Sachs disease is a hereditary disorder resulting from a deficiency of the enzyme hexosaminidase. Signs and symptoms can include blindness, delayed development, seizures and paralysis.

Tear film

The tear film is the thin layer of fluid coating the front of the eye, made up of three components: an inner mucin layer that adheres tears to the ocular surface, a middle aqueous (watery) layer that provides moisture and an outer lipid (oily) layer that limits evaporation. A healthy, stable tear film is essential for clear vision and ocular comfort.

Tear break-up time (TBUT)

Tear break-up time is a clinical test that measures the stability of the tear film. After a small amount of fluorescein dye is placed on the eye, the patient is asked not to blink while the eye doctor observes the tear film at the slit lamp. The number of seconds between the last blink and the first appearance of a dry spot is recorded as the tear break-up time. A short TBUT can indicate evaporative dry eye disease, often related to meibomian gland dysfunction. Non-invasive imaging-based methods of measuring tear film stability are also available.

Temple

Temple refers to the "arm" of a pair of glasses, running from the ear to the lens area.

Temporomandibular joint disorder (TMJ or TMD)

Temporomandibular joint disorder is a condition affecting the jaw, jaw joint and surrounding muscles that control jaw movements such as chewing, swallowing and speaking. It can cause pain, swelling and difficulty with jaw movement. Other symptoms can include toothaches, headaches and ringing in the ear (tinnitus). Causes can include jaw or facial injury, teeth grinding or clenching, stress, and osteoarthritis or rheumatoid arthritis.

Thrombosis

Thrombosis refers to a blood clot. This coagulation of the blood can occur anywhere in the circulatory system, including the heart, arteries, veins and capillaries.

Thyroid eye disease (TED)

Thyroid eye disease, also called Graves’ ophthalmopathy, is an autoimmune condition in which inflammation affects the tissues around and behind the eye. Symptoms include eye bulging, eyelid retraction, double vision, redness, dryness and, in severe cases, vision loss from compression of the optic nerve. TED is most commonly associated with Graves’ disease but can also occur with other thyroid conditions. Treatment may include lubrication, prism lenses, immunosuppressive therapy (including teprotumumab), and eyelid or orbital surgery.

Titanium

Titanium is a type of metal alloy. Eyeglasses made of titanium are often lightweight, durable and hypoallergenic.

Tonic pupil

Tonic pupil, also known as Adie’s tonic pupil, is a dilated pupil that reacts sluggishly to light. It can result from damage to the ciliary ganglion from trauma, viral infections or other causes. The cause is sometimes unknown.

Tonometry

Tonometry is the measurement of pressure inside the eye (intraocular pressure). It is a routine part of eye exams and a key tool in detecting and managing glaucoma. Techniques include Goldmann applanation tonometry, non-contact (“air puff”) tonometry and handheld devices.

Toric

Toric refers to a lens design with two different optical powers at right angles to each other, used for the correction of astigmatism. Toric contact lenses and toric intraocular lenses (IOLs) are examples of this type of lens design.

Toxocariasis

Toxocariasis is an infection caused by Toxocara worms, which are typically found in cat and dog intestines. The form found in the eyes, ocular larva migrans, can cause vision loss.

Toxoplasmosis

Toxoplasmosis is an infection caused by the Toxoplasma parasite, often from undercooked meat or contact with feces. It may occur in people with compromised immune systems. Signs and symptoms are typically flu-like and can include swollen lymph nodes and muscle aches. Ocular toxoplasmosis can cause inflammation of the eye's interior, leading to uveitis.

Trabecular meshwork

The trabecular meshwork is the porous, spongy tissue within the drainage angle of the anterior chamber of the eye, through which the aqueous humor exits the eye.

Trabeculectomy

Trabeculectomy is a surgical procedure that lowers intraocular pressure by creating a new drainage channel for aqueous humor to leave the eye, typically protected under a thin flap in the sclera. The drained fluid collects under the conjunctiva, forming a bleb. Trabeculectomy is used in moderate-to-advanced glaucoma when medications and laser have not adequately controlled pressure.

Trachoma

Trachoma is a chronic bacterial infection that affects the eyelid and cornea. While the condition commonly spreads through contact with eye discharge from an infected person, it can also be transmitted by flies. Over time, the eyelid can become scarred and turn inward. If this occurs, the eyelashes may rub against the eye and cornea, causing damage. Trachoma can potentially lead to vision impairment if left untreated.

Transient ischemic attack (TIA)

Often called a "ministroke," a TIA is a short-lived blood clot-induced blockage of the blood supply to the brain. Vision may be blurred, but other signs and symptoms may include numbness on one side of the body, slurring of speech, dizziness and paralysis.

Trauma

Trauma often refers to injury, such as from being poked in the eye or hit in the head. Depending on the type of trauma, signs and symptoms can include blurred vision, a bulging eye, burning, double vision, dry eyes, floaters, light sensitivity, pain or discomfort of the eye or around the eye, swelling, a pupil that is dilated or unresponsive to light, vision loss, limited eye or lid movement, ptosis, an iris defect and an eyelid cleft.

Trichiasis

Trichiasis is a condition in which the eyelashes grow inwardly (towards the eye).

Trichotillomania

Trichotillomania is a disorder characterized by compulsive hair or eyelash pulling. The exact cause is not known.

Trifocal

Trifocal refers to a lens design that has three focal areas: a lens for close work or reading, a lens for mid-distance viewing or at arm's length, and a lens for faraway viewing or driving.

20/20 vision

Many eye care practitioners consider 20/20 vision to be the average visual acuity for humans, but some people can see as well as 20/15 or even 20/10. People with 20/40 vision can see clearly at 20 feet what people with 20/20 vision can see clearly at 40 feet. In most of the United States, 20/40 is the lowest uncorrected acuity required for a driver's license.

Ultraviolet (UV) light

Ultraviolet light is the invisible part of the light spectrum, where light rays have wavelengths shorter than the violet end of the visible spectrum and longer than that of X-rays. UVA, UVB and UVC rays are harmful to your eyes and skin.

Uvea

The uvea is the middle layer of the eye, below the limbus. It consists of the iris, ciliary body and choroid. 

Uveitis

Uveitis is inflammation of the uvea. In many cases, the cause is unknown, but infectious or immunological systemic disorders can cause the condition. Signs and symptoms vary depending on where in the uvea the inflammation occurs, but they can include mild to severe eye pain, redness, light sensitivity, blurred vision and floaters. Tearing, a pupil that responds poorly to light or squinting may also occur. Specific types of uveitis include iritis, iridocyclitis, cyclitis, pars planitis and choroiditis.

Vascular birthmark

A vascular birthmark is a pink, red or purple mark (flat or slightly raised). It is typically located on the face or neck and is caused by a malformation of blood vessels. Common types of vascular birthmarks include capillary hemangiomas ("stork bites" or "angel's kisses") and port-wine stains.

Vascular problem

This refers to a problem with the body's vascular system (i.e., blood vessels, arteries), such as hardening of the arteries, high blood pressure, a clot, an aneurysm or an embolus. These problems can sometimes affect the eyes, resulting in such signs and symptoms as blurred vision, a bulging eye, double vision, eye pain or discomfort, a red or pink eye, eyelid swelling and vision loss.

Vector

In gene therapy, a vector is an agent, such as a modified virus, used to carry and transport genetic material that is then transferred into recipient cells in an organism.

Vergence disorder

Vergence refers to the eyes' ability to turn either inward (convergence) or outward (divergence). Convergence insufficiency is the more common vergence disorder. The exact causes are unknown. Signs and symptoms can include double vision, eye strain, fatigue, headache, squinting and difficulty concentrating (particularly while reading).

Vernal keratoconjunctivitis (VKC)

Vernal keratoconjunctivitis is a severe, chronic allergic eye disease that typically affects children and adolescents — more often boys — and is more common in warm, dry climates. It is characterized by intense itching, light sensitivity, ropy mucous discharge, large cobblestone-like papillae on the upper inner eyelid and, in some cases, corneal involvement that can threaten vision. Symptoms often worsen seasonally. Treatment may include allergen avoidance, cold compresses, and prescription antihistamine, mast cell stabilizer, anti-inflammatory or immunomodulator eye drops.

Vision therapy

Vision therapy is a customized program of supervised activities prescribed by an eye doctor to develop or improve visual skills and the way the eyes work together with the brain. It is most commonly used to treat binocular vision disorders such as convergence insufficiency, certain forms of strabismus and amblyopia, accommodative dysfunction and visual processing issues. Therapy is typically conducted in an office setting with at-home reinforcement, and may use lenses, prisms, occluders and other tools.

Visual acuity

Visual acuity refers to the sharpness of vision, usually as measured with the use of a Snellen eye chart. 20/20 is considered normal visual acuity, though some people can have even sharper vision (such as 20/15 or 20/10).

Visual field

The visual field is the entire area a person can see when looking straight ahead, including central and peripheral vision. Visual field testing maps blind spots and areas of reduced sensitivity and is essential in monitoring glaucoma, neurological conditions, and other diseases that affect the optic nerve or retina.

Vitrectomy

Vitrectomy is a surgical procedure in which some or all of the vitreous gel is removed from the eye. It is used to treat a range of conditions, including vitreous hemorrhage, retinal detachment, macular hole, epiretinal membrane and advanced diabetic eye disease. The vitreous is replaced with a saline solution, gas bubble or silicone oil, depending on the condition being treated.

Vitrector

A vitrector is a tiny, motorized cutting instrument used to remove the eye's gel-like vitreous during a vitrectomy.

Vitreous body

The vitreous body, also called the vitreous humor, is the part of the eye between the lens and the retina, containing a clear jelly called the vitreous humor.

Vitreous detachment

Vitreous detachment, or posterior vitreous detachment, refers to the separation of the vitreous from the retina, commonly caused by age-related vitreous shrinkage. Eye floaters are a typical symptom, but some people also experience flashes of light as the vitreous tugs or causes traction on the retina prior to complete separation.

Vitreous hemorrhage

Vitreous hemorrhage occurs when blood leaks into the vitreous from nearby parts of the eye, such as from leaking retinal blood vessels. Causes can include diabetic retinopathy, trauma, a retinal tear or detachment, vitreous detachment and retinal vascular occlusion (a blockage in the retina's vascular system). Symptoms can include sudden blurring or loss of vision and new floaters.

Wavefront

In optics, wavefront refers to the surface connecting similar points on adjacent waves of light among an essentially infinite number of waves traveling together from a light source. The simplest form of a wavefront is called a plane wave, where all light rays are parallel to each other. Distortions of visual wavefronts caused by irregularities of the optical system of the eye are called higher-order aberrations (HOAs). These wavefront distortions can be measured at the plane of the pupil by instruments called aberrometers, which document HOAs in the form of a topographical map. These maps can then be used to correct HOAs with customized, wavefront-guided LASIK.

Wear schedule

Wear schedule refers to how long you wear your contact lenses: either daily wear (you remove the lenses each night) or extended wear (you may wear them continuously or sleep with them in). It's important to differentiate between wear schedule and replacement schedule — that is, how often you discard and replace your lenses.

White dot syndrome

White dot syndrome is one of a group of inflammatory conditions that are characterized by white dots in the retina and choroid. You may also hear these syndromes called by their specific names, such as acute posterior multifocal placoid pigment epitheliopathy, multiple evanescent white dot syndrome, birdshot chorioretinopathy or multifocal choroiditis and panuveitis. In some cases, the cause is unknown; in others, it's believed to be an autoimmune disorder. Symptoms can include blurred vision, loss of color vision, floaters, light sensitivity, metamorphopsia and vision loss.

Wraparound

Also called "wrap" for short, this is a type of eyeglass or sunglass frame that curves around the head, from the front to the side. Wraparound sunglasses may offer extra sun and wind protection at the sides. Many frames cannot be fit with prescription lenses because the curvature causes optical distortion. However, some of the newer styles have been engineered to overcome this challenge.

Xanthelasma

A xanthelasma is a yellow, fatty spot or bump on the inner corner of either the upper eyelid, the lower eyelid or both. It is often caused by a lipid disorder such as high cholesterol.

YAG laser capsulotomy

YAG (yttrium-aluminum-garnet) laser capsulotomy is a quick, in-office laser procedure that treats posterior capsular opacification — a common, painless clouding of the lens capsule that can develop months or years after cataract surgery. The laser creates a small opening in the cloudy capsule behind the artificial intraocular lens, typically restoring vision within hours.

Zeaxanthin

Zeaxanthin is a pigmented substance (carotenoid) found in yellow or orange plants, such as corn and squash, or in dark green, leafy vegetables. Zeaxanthin is being investigated for a possible association with promoting healthy vision.

Zonules

The zonules, also called the zonule of Zinn or the suspensory ligament of the lens, are a network of fine fibers that connect the eye’s crystalline lens to the ciliary body, holding the lens in place and transmitting the focusing action of the ciliary muscle.

Zylonite (Zyl)

Zylonite,  or cellulose acetate, is a lightweight plastic commonly used in eyeglass frames. It often appears in laminated form, with layers in different colors, but it can also be made in mottled patterns to imitate natural tortoiseshell or other patterns.