Glossary of Eye Care Terms - C-D
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.
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.
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. Read more about color blindness.
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. Please read our articles on computer vision syndrome to learn more.
cone - A photosensitive receptor in the retina that helps you to see color. Learn more about the retina.
conjunctiva- The thin, normally clear, moist membrane that covers the "white" of the eye (sclera) and the inner surface of the eyelids. Learn more about the conjunctiva of the eye.
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. For tips on what to do about contact lens discomfort, please read our article.
contrast sensitivity - The ability of the eye to detect the line of demarcation between an object and its background or an adjacent object. Learn more about contrast sensitivity.
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 portion of the front surface of the eye that allows light to enter the eye for sight. The cornea provides most of the focusing power of the eye. Learn more about the cornea of the eye.
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. Learn more about corneal abrasions.
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. Learn more about corneal collagen crosslinking.
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 ecstasia - A progressive thinning of the middle layer of the cornea (stroma) causing the front surface of the eye to bulge forward. Ectasia causes irregular astigmatism, higher-order aberrations, distorted vision and corneal scarring. Causes of corneal ectasia include keratoconus, other corneal diseases, and (infrequently) LASIK surgery. Also called keratectasia or keratoectasia.
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. Read more about corneal inserts.
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. Read more about Bell's palsy.
crossed eyes - Type of strabismus (a misalignment of the eyes) where one or both eyes point inward, toward the nose. Learn more about crossed eyes.
culture - In medical terminology, tests of samples such as blood or body material to determine possible presence of bacteria or other substances.
cycloplegia - Paralysis of the eye's ciliary (focusing) muscle and power of accommodation. A cycloplegic refraction is one that is performed after eye drops are placed in the eye to dilate the pupil and temporarily paralyze the accommodative ability of the eye that might mask some degree of refractive error.
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. Learn more about dacryoadenitis.
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.
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. For more information about diabetes and related eye disease, please read our article on diabetic retinopathy.
diabetic vitreous hemorrhage - Bleeding into the gel-like (vitreous) interior of the eye, originating from blood vessels weakened by diabetes. Because blood in the vitreous can block vision, a vitrectomy may be required.
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 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 - Also called double vision (see below).
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. Learn more about drooping eyelids.
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.
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 articles on dry eye syndrome and relief for dry eyes.
dry skin - Skin that is generally dry may include dry eyelids.
Page published in April 2019
Page updated in August 2021