Glossary of Eye Care Terms
aberrometer 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 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 such as LASIK and PRK, the excimer laser ablates, or removes, tissue from the cornea.
accommodation Eye's ability to automatically change focus from seeing at one distance to seeing at another.
accommodation disorder Accommodation refers to the eye's ability to automatically change focus from seeing at a distance to seeing at near. 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 everyone if they become old enough, since its causes relate to the aging of the eyes.
acetate Type of plastic often used in eyeglass frames.
acrodermatitis enteropathica Rare inherited condition characterized by dermatitis, hair loss, diarrhea and zinc deficiency. The most common eye symptom is light sensitivity, but conjunctivitis and other symptoms may also be present.
AK (astigmatic keratotomy) Procedure in which a surgeon cuts the cornea so that it is more spherical when it heals, thus reducing astigmatism.
albinism Condition where a person or animal lacks pigment. Albinos' eyes often have very light blue or pink irises and a pink pupil (due to lack of pigment inside the back of the eye). Visual symptoms include light sensitivity, nystagmus, blurred vision, vision loss and strabismus.
allergen Any substance (such as pollen, mold, dust or animal dander) that causes allergy symptoms in sensitive individuals.
allergy Reaction of the body's immune system to a foreign substance (e.g., pollen, animal dander, etc.). When the eyes are affected, the most common symptoms are redness, itching, chemosis, tearing, swollen eyelids and stickiness. Read more about allergies and the eyes.
ALS (Lou Gehrig's disease) Amyotrophic lateral sclerosis is a disease that causes degeneration of neurons in the brain stem and spinal cord. Voluntary muscles atrophy and eventually become paralyzed.
amaurosis fugax 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.
amblyopia Also called lazy eye. Undeveloped central vision in one eye that leads to the use of the other eye as the dominant eye. Strabismus is the leading cause, followed by anisometropia. There are 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.
AMD or ARMD (age-related macular degeneration) Disorder characterized by the gradual loss of central vision due to a damaged macula (which is made up of retinal cones necessary for sight). Read more about macular degeneration.
angiogenesis The formation of new blood vessels in the body. Also see neovascularization.
aniridia Absent or partially absent iris, typically congenital. Additional symptoms include poor vision and photophobia.
anisocoria Unequal pupil size. Causes include glaucoma, head or eye trauma, an intracranial tumor, infection of the membranes surrounding the brain and previous intraocular surgery. A small percent of the population has unequal-sized pupils naturally (without any known cause).
anisometropia Condition where the eyes have a significantly different refractive power from each other, so the prescription required for good vision will be different for each eye.
anophthalmos Absence of one or both eyes. Anophthalmos may be congenital or due to trauma, infection or other causes. Symptoms include reduced depth perception and peripheral vision.
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.
ANSI Z87.1-2003 Standard The American National Standards Institute's Practice for Occupational and Educational Eye and Face Protection; eyewear that meets this standard is considered safer than eyewear that does not.
anterior chamber Part of the eye behind the cornea and in front of the iris and lens.
antibody A protein activated by the body's immune system that fights infection, toxins and other foreign substances.
antioxidant Substance that inhibits oxidation and can guard the body from damaging effects of free radicals. Molecules with one or more unpaired electrons, free radicals can destroy cells and play a role in many diseases. Antioxidant vitamins include B, C and beta carotene. Antioxidants may help prevent macular degeneration and other eye diseases; many studies are in progress. More about nutrition and the eyes.
anti-reflective coating (AR coating) Thin layer(s) applied to a lens to reduce the amount of reflected light and glare that reaches the eye. Read more about anti-reflective lenses and eyeglass lens coatings.
apheresis A process in which blood is drawn outside the body, certain compounds are removed, and the blood is returned to the body. The technique has various applications, including: harvesting of needed components such as plasma or white blood cells; and removing harmful components such as large proteins, in order to treat the dry form of age-related macular degeneration (AMD). For more information, please see our article on age-related macular degeneration.
aqueous humor The clear, watery fluid that fills the space between the cornea and 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 Small, irregular pupil, usually caused by syphilis. Argyll Robertson pupils do not respond to light.
aspheric Not quite spherical. Aspheric eyeglass lenses are popular among people who have strong prescriptions because they are thin and lightweight, and reduce distortion and eye magnification. Aspheric contact lenses can work as a multifocal, or to correct a single-vision problem like astigmatism.
asteroid hyalosis A benign condition that creates suspended particles within the eye's interior, observable by an eye doctor during an exam. Floaters are not usually associated. 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. The condition is not well understood, but appears associated with aging.
astigmatism Condition in which the cornea's curvature is asymmetrical (the eye is shaped like a football or egg instead of a baseball); light rays are focused at two points on the retina rather than one, resulting in blurred vision. Additional symptoms include distorted vision, eye strain, shadows on letters, squinting and double vision. Read more about astigmatism.
atopy Type of allergy where levels of the antibody immunoglobin E are increased; atopy includes rhinitis, asthma, hay fever and eczema.
aura A sensation 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 Glasses with a large, upside-down teardrop shape like the sunglasses that aircraft pilots have worn traditionally; they usually 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. They offer a distinctly retro or vintage look.
band keratopathy Opacity of the corneal stroma and Bowman's membrane. Symptoms include vision loss and foreign body sensation.
benign essential blepharospasm Neurological disorder causing involuntary muscle contractions, with eyelid muscle spasms.
best corrected visual acuity (BCVA) The best 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.
beta blocker Drug that widens or dilates blood vessels, thus enabling more normal flow of blood. Topical beta blockers applied as eye drops also can lessen fluid production and lower internal eye pressure (intraocular pressure) in eye diseases such as glaucoma to reduce the possibility of optic nerve damage. Beta blockers also are used to control high blood pressure (hypertension). Side effects can include respiratory problems.
beta titanium Titanium alloy. Beta-titanium eyeglass frames are both strong and flexible.
binocular vision Ability of both eyes to work together to achieve proper focus, depth perception and range of vision.
biocompatible Able to coexist with living tissues without harming them. For example, artificial lenses are designed to be biocompatible with tissue inside the eye so they won't cause a toxic or immunilogical response that would harm the eye.
bioptic telescopic lenses (BTL) Devices attached to glasses that provide extreme magnification, typically used for driving. For people with low vision who are qualified, telescopic lenses are attached above the driver's line of sight to help magnify objects such as road signs.
blepharitis Inflammation of the eyelid(s), typically around the eyelashes. Various types of dermatitis, rosacea and allergic reactions can cause blepharitis. Symptoms include a red or pink eyelid, crusty lid 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 Excessive, drooping eyelid skin caused by recurring swelling. Blepharochalasis typically occurs in young people.
blepharoconjunctivitis Inflammation of the eyelid and conjunctiva. Infections and allergic reactions are among the causes. Symptoms 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.
blepharospasm 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 produce a functional blindness since the patient can't open his or her eyes long enough to function visually.
blue light Found in the light spectrum next to violet, which has the shortest wavelength of visible light. Also known as High Energy Visible (HEV) light, blue light has been linked to eye damage and diseases such as age-related macular degeneration.
botulism Serious illness from a toxin produced by Clostridium bacteria (usually Clostridium botulinum). Infant botulism and food-borne botulism are the most common forms in the United States. Symptoms include double vision, blurred vision, ptosis, muscle weakness, difficulty speaking, difficulty swallowing, difficulty breathing and nausea.
Bowman's membrane Corneal layer between the epithelium and the stroma.
bridge The part of an eyeglass frame that extends across the nose.
cable temple A type of "arm" of a pair of glasses that wraps around the ear, to keep the eyewear firmly attached. Often used in eyeglasses for infants and toddlers.
canaliculus Tiny, tube-shaped passageway in the body. In the eye, canaliculi are tiny channels at the beginning of tear ducts through which tears drain until they exit into the nose.
canaliculitis Inflammation of a tear duct (or ducts), caused by a bacterial infection. Symptoms of this disorder include discharge, a red or pink eye and a swollen upper or lower eyelid near the nose.
capsulotomy In cataract surgery, an incision into the capsule that contains the natural crystalline lens of the eye, in order to remove and replace the natural lens with an artificial one.
carotenoid A pigmented substance that adds color such as red, orange, or yellow to plants. Carotenoids have antioxidant properties that protect cells against damage from free radicals, unstable atoms that can interact with and break down healthy tissue in different parts of the body including eyes.
cataract Clouding of the natural lens of the eye, usually caused by aging in conjunction with other risk factors, such as exposure to the sun's UV rays, smoking, steroid intake and diabetes. Symptoms include blurred vision, glare, halos around lights, colors that are less bright, a cloudy spot in your vision and, sometimes, temporary vision improvement. Read more about cataracts and cataract surgery.
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 include tumors, aneurysms and clots. Typical symptoms include ophthalmoplegia, chemosis and a bulging eye. You may also experience a red eye and vision loss.
central island Refractive surgery complication in which the laser leaves an "island" of corneal tissue in the concave ablation zone. Symptoms include double vision and distortion. Read more about potential complications of LASIK and other kinds of refractive surgery.
central serous retinopathy Disorder in which fluid collects under the central retina (macular area) and disrupts central vision. The cause is unknown. Symptoms include blurred central vision and metamorphopsia. Some patients also develop floaters.
cerebral cortex Outer portion of the brain where complex functions including certain vision processes take place.
cerebral palsy Group of chronic conditions caused by brain damage before birth or during infancy. The many effects may 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 small bump on the eyelid caused by an obstructed meibomian gland. Additional symptoms include light sensitivity, tearing and eyelid swelling. Chalazia are usually not painful unless they become infected. Read more about chalazia.
chemosis Conjunctival swelling that is often caused by an allergy.
choroid Layers of blood vessels located between the sclera (white of the eye) and the retina; they provide nourishment to the back area of the eye.
choroidal neovascularization Abnormal growth of new blood vessels in the choroid. Choroidal neovascularization is commonly associated with macular degeneration, but it can occur as a result of other eye conditions as well. Symptoms include vision loss and metamorphopsia.
chromosomes 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 Part of the eye between the iris and the choroid; the three form the uvea. The ciliary body's main functions are accommodation, aqueous humor production and holding the lens in place.
CK (Conductive Keratoplasty) Procedure in which a surgeon uses radio waves to heat collagen in the cornea's periphery to shrink it and reduce hyperopia (farsightedness). CK is also used to treat presbyopia. Read more about CK.
clip-on Type of glasses that attaches to your regular glasses, such as clip-on sunglasses.
CMV retinitis (cytomegalovirus retinitis) Serious eye infection usually found in those with immune problems, such as AIDS patients; symptoms include floaters, blind spots, blurry vision and vision loss.
cohort A term used in clinical studies to define a set of people who have something in common such as similar backgrounds, experiences, and/or health problems.
collagen Fibrous protein 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 Cleft, usually due to incomplete embryologic development in utero. An iris coloboma is the most common 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; only iris and eyelid colobomas are visible with the naked eye. Additional symptoms such as poor vision may occur, but are not readily apparent from a parent's perspective.
color blindness Partial or total inability to distinguish specific colors. Color blindness is inherited, and is much more common in men than in women.
computer vision syndrome Collection of problems, mostly eye- and vision-related, associated with computer use. Symptoms include eye strain, dry eyes, blurred vision, red or pink eyes, burning, light sensitivity, headaches and pain in the shoulders, neck and back.
cone A photosensitive receptor in the retina that helps you to see color.
conjunctiva The thin, normally clear, moist membrane that covers the "white" of the eye (sclera) and the inner surface of the eyelids.
conjunctivitis Inflammation of the conjunctiva, characterized by a pink eye. The cause is either infectious or allergic, though the term "pink eye" is commonly used for any type of conjunctivitis. Other symptoms include burning, discharge, dryness, itching, light sensitivity, eye pain or discomfort, stickiness, tearing and chemosis.
contact lens drops Eye drops 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. 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 The ability of the eye to detect the line of demarcation between an object and its background or an adjacent object.
convergence Eyes' ability to turn inward. People with convergence insufficiency have trouble (eye strain, blurred vision, etc.) with near tasks such as reading.
cornea The clear part of the eye covering the iris and pupil; it lets light into the eye, permitting sight.
corneal abrasion A loss of the epithelial layer of the cornea, typically due to minor trauma (contact lens trauma, a sports injury, dirt or another foreign body, etc.). Symptoms include blurred vision, foreign body sensation, grittiness, light sensitivity, eye pain or discomfort, a red or pink eye and tearing.
corneal arcus 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 commonly seen in older adults is sometimes called arcus senilis. Corneal arcus seen in younger individuals is associated with high cholesterol.
corneal collagen crosslinking A medical 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 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 include blurred vision, foreign body sensation, light sensitivity, eye pain or discomfort and vision loss.
corneal edema Swelling of the eye's cornea; causes include intraocular surgery, corneal dystrophies, high intraocular pressure and contact lens complications. Symptoms include vision loss, halos around lights, a white or cloudy spot on the eye, photophobia, eye pain and foreign body sensation.
corneal erosion Recurrent breakdown of the corneal epithelium, typically caused by a previous corneal abrasion or by map-dot-fingerprint dystrophy. Symptoms include blurred vision, foreign body sensation and eye pain or discomfort.
corneal implants Devices (such as rings or contacts) placed in the eye, usually to correct vision.
corneal opacity A cloudy spot in the cornea, which is normally transparent. Causes include corneal scar tissue and infection. Symptoms include halos around lights, photophobia, vision loss and a white or cloudy spot on the eye.
corneal refractive therapy (CRT) The use of specially designed gas permeable contact lenses to reshape the cornea during sleep and temporarily correct myopia (with or without mild-to-moderate astigmatism). Developed by Paragon Vision Sciences. Similar to orthokeratology.
corneal ring Type of vision correction surgery where a 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 better onto the retina so you can see better. The ring can be adjusted and even removed if desired.
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 used for evaluations related to refractive surgery, contact lens fitting and corneal disease management. It is 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 An infected corneal abrasion. Frequently found in extended wear contact lens wearers. A corneal ulcer is an ocular emergency. Symptoms include light sensitivity, eye pain or discomfort, a red or pink eye, a white or cloudy spot on the eye and tearing.
cranial nerve One of the 12 pairs of nerves that go from the brain to other parts of the head. Those that affect the eyes and vision are the second cranial nerve (optic nerve), third (oculomotor), fourth (troclear), 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 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 with such symptoms 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 include head trauma, diabetes, tumors, aneurysms, infarction (tissue death) and more.
In most cases, the cause of paralysis of the seventh cranial nerve is unknown (termed "Bell's palsy"). Symptoms 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 Type of strabismus (a misalignment of the eyes) where one or both eyes point inward, toward the nose.
culture In medical terminology, tests of samples such as blood or body material to determine possible presence of bacteria or other substances.
cystoid macular edema (CME) Swelling of the eye's macula, caused by an excessive amount of fluid.
dacryoadenitis Inflammation of the tear gland, typically caused by a viral or bacterial infection. Symptoms include a dry eye, a red or pink eyelid, swelling of the lid or around the eyes and ptosis.
dacryocystitis Inflammation of the nasolacrimal (tear) sac, typically caused by dacryostenosis. Symptoms include discharge, a sticky eye, eye pain or discomfort, a red or pink eye, swelling around the eye and tearing.
dacryostenosis Blocked tear duct, which is characterized by a lot of tearing; you may also have a discharge or a sticky eye.
daily wear These soft contact lenses are worn every day for six months up to a couple of years. They require daily cleaning and disinfecting, as well as a periodic enzymatic soak (usually once a week).
decentered ablation Also called decentration. Refractive surgery complication in which the laser is not centered on the pupil when it removes tissue. Symptoms include glare, double vision and halos.
dermatochalasis Excessive, drooping eyelid skin caused by a loss of elasticity in aging skin.
Descemet's membrane Corneal layer between the stroma and the endothelium.
diabetic maculopathy Eye disease related to diabetes that creates 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. Another form of diabetic eye disease is diabetic retinopathy.
diabetic retinopathy Leaking of retinal blood vessels in advanced or long-term diabetes, affecting the macula or retina. Most people have no symptoms at first, but can develop blurred near vision, double vision, floaters, retinal/vitreous hemorrhages and metamorphopsia. In later stages, you can also suffer vision loss.
diabetic vitreous hemorrhage Bleeding into the gel-like (vitreous humor) interior of the eye, originating from blood vessels weakened by diabetes. Because blood in the vitreous can block vision, a vitrectomy may be required. For more about diabetes and eye disease, read our article about diabetic retinopathy.
diode 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 Unit of measure for the refractive (light-bending) power of a lens; eye care practitioners use it in eyeglass 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 Also called double vision. When two images of the same object are perceived by one or both eyes. Read our article on double vision and also review eye symptoms for a list of conditions that can cause double vision.
disc hemorrhage Bleeding in the back of the eye, occurring usually at the edge of the optic disc (the area where the optic nerve attaches to the retina). Also called splinter hemorrhage. Though disc hemorrhages usually disappear in a few months, they are taken seriously because 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 Generally refers to eyesight for tasks beyond arm's length, such as driving, watching television and movies, participating in sports, etc. Distance vision is tested during an eye exam with a standard eye chart at a distance of approximately 20 feet. See also: near vision and intermediate vision.
Dk/t Dk is the oxygen permeability of a contact lens material; t is the thickness of the contact lens design. Dk/t is a measurement of a contact lens's oxygen transmissibility.
double vision Also called diplopia. When two images of the same object are perceived by one or both eyes. Read our article on double vision and also review eye symptoms for a list of conditions that can cause double vision.
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 Also called ptosis. Condition in which the upper eyelid(s) only sag. It can be present at birth or caused by a later 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. 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 small, yellow or white deposit in the eye. "Drusen" is the plural form of the word. Drusen are sometimes signs of macular degeneration.
dry eye Lack of sufficient lubrication and moisture in the eye. Most dry eye complaints are temporary and easily relieved; dry eye syndrome, also called keratoconjunctivitis sicca, is chronic and needs more advanced treatment by an eye care practitioner. Read our article on dry eye syndrome.
dry eye syndrome Chronic dryness due to 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 symptoms include foreign body sensation, eye pain or discomfort, burning, grittiness, itching, light sensitivity, frequent blinking, a red or pink eye and tearing. Read our article on dry eye syndrome.
dry skin Skin that is generally dry may include dry eyelids.
ectropion An abnormal turning out of an eyelid, typically the lower one, which exposes the inner, conjunctival side of the eyelid; usually due to aging. Additional symptoms include eye or lid pain or discomfort, a red or pink eye or eyelid and overflow tearing.
edema Accumulation of an excessive amount of watery fluid, which causes swelling.
emmetropia The condition of an eye with normal vision, meaning that light rays correctly are focused at the inner back of the eye (retina) where images are processed.
endophthalmitis Inflammation of the interior of the eye, typically caused by an infection from eye surgery or trauma. Endophthalmitis is an ocular emergency. Symptoms include floaters, light sensitivity, eye pain or discomfort, a red or pink eye and vision loss.
endothelium The cornea's inner layer of cells.
enophthalmos The sinking of the eye into the socket. Causes include development problems in utero, trauma and inflammation.
endpiece At left and right of the front of an eyeglass frame, the part that attaches to the temples, usually but not always with a screw mounting.
entropion An abnormal turning in of an eyelid, which causes the lashes to rub on the ocular surface; usually due to aging. Additional symptoms include eye or lid pain or discomfort, foreign body sensation, a red or pink eye, itching, tearing and vision loss.
environmental condition Air pollution, wind and bright light can irritate your eyes and cause symptoms such as burning, dryness and tearing.
enzymatic cleaner A cleaner that removes protein deposits and other debris from contact lenses. It's 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.
epiretinal membrane Thin layer of scar tissue on the retina; also called a macular pucker. Epiretinal membranes have a variety of causes, including vitreous detachment, but the cause is often unknown. In its early stages, an epiretinal membrane is often asymptomatic, but some people have blurred vision. You may also develop metamorphopsia.
episclera Outer layer of the eye's sclera that loosely connects it to the conjunctiva.
episcleritis Inflammation of the episclera. The cause is usually unknown, but episcleritis may be associated with some systemic (e.g., autoimmune) diseases. Symptoms include a red or pink eye, eye pain or discomfort, light sensitivity and tearing.
epithelial ingrowth LASIK complication in which epithelial cells grow under the LASIK flap. Often it is temporary and doesn't usually affect vision. But sometimes it does affect vision and requires the flap to be lifted and the cells removed.
epithelium The cornea's outer layer of cells.
esotropia When one or both eyes point inward, so the eyes are "crossed." This is one type of strabismus.
excimer laser An instrument that uses shorter wave (ultraviolet) light to vaporize and remove tissue from the eye's surface during vision correction procedures.
exotropia When one or both eyes point outward; also called "walleyed." This is one type of strabismus.
extended wear Currently, these contact lenses are FDA-approved to be worn without removal for up to seven days (or 30 days in the case of one brand), meaning some people will be comfortable sleeping with them in their eyes. Thirty-day contact lenses are sometimes referred to as "continuous wear."
eye care practitioner Optometrists (ODs) and ophthalmologists (MDs) both practice eye care, but in different, though often overlapping, areas: In the United States, ODs (Doctors of Optometry) examine eyes for both vision and health problems, prescribe eyeglasses, prescribe and fit contact lenses, and treat some eye conditions and diseases. ODs attend four years of optometry school after attaining their BS or BA college degree. MDs are medical doctors who specialize in the eyes. They examine eyes, treat disease, perform surgery, and prescribe glasses and contacts. Like other physicians, they complete a BS or BA degree, attend four years of medical school, and complete a residency program in their practice specialty. Both ODs and MDs often pursue further subspecialty fellowship training, and they take additional continuing education courses during their careers in order to stay up to date and to maintain state and national board certifications. Other non-doctor eye care practitioners include paraoptometrics, contact lens technicians, and opticians, whose training and continuing education requirements can differ depending on the state in which they practice.
eye tumor A growth or mass that occurs in or next to the eye. Specific tumors, both benign and malignant, include the dermoid cyst, capillary hemangioma, cavernous hemangioma, choroidal melanoma, retinoblastoma, rhabdomyosarcoma and lymphoma. The cause is dependent on the type of tumor you have. Symptoms can include blurred vision; a bulging eye; double vision; floaters; foreign body sensation; pain or discomfort in the eye, the lid 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; and an iris defect.
farsightedness Also called hyperopia. To farsighted people, near objects are blurry, but far objects are in focus.
FDA (Food & Drug Administration) 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 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 Condition that produces long-term pain all over the body and/or 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, the eye's ability to maintain gaze upon an object.
flap and zap Slang for LASIK.
floaters A dark or gray spot or speck that passes across your field of vision and moves as you move your eye. Floaters are very common and may look like clouds, strands, webs, spots, squiggles, wavy lines or other shapes. As your eye ages, the gelatinous vitreous humor begins to liquefy in the center of the gel. Floaters are caused by the undissolved vitreous humor that floats in the liquid vitreous. Sometimes, a "shower of floaters" is a sign of a serious condition, particularly if you also see flashes of light. Read our spots and floaters article.
fluorescein Compound that becomes a bright, fluorescent yellow-green when in contact with alkaline substances. A fluorescein dye solution can help eye doctors see corneal lesions or conduct tests for eye dryness.
fluorescein angiography An imaging test that involves first injecting fluorescent yellow-green dye into the veins. When the dye reaches interior regions of the eye, it provides opportunity for high contrast photography or other imaging of blood vessels. Fluorescein angiography particularly is 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 Surgical tool shaped like tongs, used for gripping.
foreign body Something in or on the eye that doesn't belong there. Symptoms include 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 Sensation that something is in your eye.
fovea A depression in the retina that contains only cones (not rods), and that provides acute eyesight.
free radicals 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 Also called planned replacement. Technically, this is any contact lens that is thrown away after a moderately 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.
fungal keratitis eye infection The source of a 2006 outbreak of fungal eye infections among contact lens wearers is a fungus known as Fusarium, found in places such as soil, water, and organic matter including plants. Ordinarily, it is rare for this fungus to invade and damage the eye. But symptoms can be severe, and if untreated, the infection may become so eye-damaging that a corneal transplant is required. For more information, please see our fungal keratitis eye infection article.
geographic atrophy Deterioration of tissue in the central portion of the retina, often associated with aging. Geographic atrophy (GA) is considered the end stage of a "dry" form of age-related macular degeneration, which occurs when tissue in the retina begins to break down and form yellowish spots known as drusen. GA can cause significant central vision loss.
glaucoma Disease characterized by elevated intraocular pressure, which causes optic nerve damage and subsequent peripheral vision loss. Most people have no initial symptoms of chronic (open-angle) glaucoma, but you can develop peripheral vision loss, headaches, blurred vision, difficulty adapting to darkness and halos around lights. Other forms of glaucoma (e.g., closed-angle glaucoma) may have additional symptoms such as eye pain, a pupil that doesn't respond to light, redness, nausea and a bulging eye.
glycemic index 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, create more stable blood sugar levels.
Graves' ophthalmology Autoimmune eye disorder usually associated with abnormalities of the thyroid gland; symptoms 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, these are the small, hard lenses made of PMMA material that many people wore in the '70s and '80s. Compared with modern soft and rigid lenses, they are less healthy to wear long-term, since the material doesn't allow oxygen to reach the surface of the eye.
hemifacial spasm Involuntary muscles twitches 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 Profuse bleeding.
heterochromia Condition where one eye is a different color from the other, or one eye is more than one color.
higher-order aberration Irregularity of the eye other than a refractive error (myopia, hyperopia or astigmatism). Higher-order aberrations sometimes affect your vision (such as decreasing contrast sensitivity), and sometimes do not. For more information, please see our higher-order aberration article.
high-index Type of 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) so the lens can be thinner.
histamine A protein that can be released as part of the body's immune system responses during an allergic reaction. Presence of histamine can lead to inflammation and swelling, which is why antihistamines often are prescribed for allergy symptoms.
histiocytosis Abnormal proliferation of histiocytes (immune system cells). Common symptoms include bone tumors and skin rashes. If histiocytosis affects the eyes, it causes bulging.
homocysteine 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. More studies are needed to verify such a link.
Horner's syndrome Condition characterized by a small pupil, ptosis and an abnormal lack of facial perspiration (all on the same side of the face); Horner's syndrome is caused by injury to the sympathetic nerves of the face.
hybrid contact lenses Contact lenses that have a central optical zone made of rigid gas permeable (GP) plastic, surrounded by an outer "skirt" of silicone hydrogel or regular hydrogel material. Hybrid contacts are designed to provide the crisp optics of GP contact lenses and wearing comfort comparable to that of soft lenses.
hyperopia Also called farsightedness. Condition in which the length of the eye is too short, causing light rays to focus behind the retina rather than on it, resulting in blurred near vision. Additional symptoms include eye strain and squinting.
hypoglycemia Abnormally low level of sugar (glucose) in the blood, which causes dizziness, hunger, shakiness and other symptoms. Skipping or delaying meals, too-small meals, or a high level of physical activity can cause hypoglycemia. Too much alcohol or certain drugs can also cause it. In diabetics, it can be caused by too much insulin.
hypotony Low intraocular pressure, often caused by eye surgery or trauma (e.g., open globe injury). Symptoms include blurred vision and eye pain or discomfort.
[Page updated April 2013]
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