GLOSSARY OF EYE TERMINOLOGY
Common terms -- symptoms, tests, treatments, surgery, diseases & conditions, anatomy -- eye doctors use. These definitions may help you understand them better.

Excerpted from Dictionary of Eye Terminology, copyright 1990-2007 by Barbara Cassin and Triad Publishing. The Dictionary's more than 5,000 entry terms define anatomy, tests, diseases and conditions, refractive errors, surgical procedures, drugs and medications, etc. Learn more about the Dictionary of Eye Terminology.

The information given is for information purposes only and is not intended to replace professional medical advice from your physician, or to diagnose or treat a health problem or disease.

These copyrighted definitions may not be used in any form without permission from the publisher.  Click here for information on licensing this glossary for your own website.

Dictionary of Eye Terminology

A / B / C / D / E / F / G / H / I / K / L / M / N / O / P / R / T / U / V / W / Y / Z

A

accommodation (uh-kah-muh-DAY-shun). Increase in optical power by the eye in order to maintain a clear image (focus) as objects are moved closer. Occurs through a process of ciliary muscle contraction and zonular relaxation that causes the elastic-like lens to "round up" and increase its optical power. Natural loss of accommodation with increasing age is called presbyopia.

after-cataract, secondary cataract. Remnants of an opaque lens remaining in the eye, or opacities forming, after extracapsular cataract removal.

age-related macular degeneration (AMD, ARMD) (MAK-yu-lur). Group of conditions that include deterioration of the macula, resulting in loss of sharp central vision. Two general types: "dry," which is more common, and "wet," in which abnormal new blood vessels grow under the retina and leak fluid and blood (neovascularization), further disturbing macular function. Most common cause of decreased vision after age 60.

ALK. See automated lamellar keratoplasty.

amblyopia (am-blee-OH-pee-uh), "lazy eye." Decreased vision in one or both eyes without detectable anatomic damage in the eye or visual pathways. Usually uncorrectable by eyeglasses or contact lenses.

Amsler grid (AM-slur). Test card; grid (black lines on white background or white lines on black background) used for detecting central visual field distortions or defects, such as in macular degeneration. x

angle, anterior chamber angle. Junction of the front surface of the iris and back surface of the cornea, where aqueous fluid filters out of the eye.

anterior chamber. Fluid-filled space inside the eye between the iris and the innermost corneal surface (endothelium).

aphakia (ay-FAY-kee-uh). Absence of the eye's crystalline lens, such as after cataract extraction.

aqueous (AY-kwee-us), aqueous humor. Clear, watery fluid that fills the space between the back surface of the cornea and the front surface of the vitreous, bathing the lens. Produced by the ciliary processes. Nourishes the cornea, iris, and lens and maintains intraocular pressure.

A-scan. Type of ultrasound, radar-like device that emits very high frequency waves that are reflected by the ocular structures and converted into electrical impulses. Used for differentiating normal and abnormal eye tissue or for measuring length of eyeball.

asthenopia (as-then-OH-pee-uh). Vague eye discomfort arising from use of the eyes; may consist of eyestrain, headache, and/or browache. May be related to uncorrected refractive error or poor fusional amplitudes.

astigmatism (uh-STIG-muh-tiz-um). Optical defect in which refractive power is not uniform in all directions (meridians). Light rays entering the eye are bent unequally by different meridians, which prevents formation of a sharp image focus on the retina. Slight uncorrected astigmatism may not cause symptoms, but a large amount may result in significant blurring and headache.

automated lamellar keratoplasty (ALK). Excision of the outer corneal layers (lamellae) with a computer controlled keratome (knife), usually as a part of a refractive keratoplasty procedure.

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B

background retinopathy. See diabetic retinopathy.

bifocals. Eyeglasses that incorporate two different powers in each lens, usually for near and distance corrections.

binocular vision. Blending of the separate images seen by each eye into one composite image.

blepharitis (blef-uh-RI-tus). Inflammation of the eyelids, usually with redness, swelling, and itching.

blind spot. Sightless area within the visual field of a normal eye. Caused by absence of light sensitive photoreceptors where the optic nerve enters the eye.

B-scan. Type of ultrasound; provides a cross-section view of tissues that cannot be seen directly. High frequency waves are reflected by eye tissues and orbital structures and converted into electrical pulses, which are displayed on a printout.

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C

cataract. Opacity or cloudiness of the crystalline lens, which may prevent a clear image from forming on the retina. Surgical removal of the lens may be necessary if visual loss becomes significant, with lost optical power replaced with an intraocular lens, contact lens, or aphakic spectacles. May be congenital or caused by trauma, disease, or age.

cataract extraction. Removal of a cloudy lens from the eye. An extracapsular cataract extraction leaves the rear lens capsule intact; with an intracapsular extraction there is complete removal of lens with its capsule, usually by cryoextraction.

central retinal artery. First branch of the ophthalmic artery; supplies nutrition to the inner two-thirds of the retina.

central retinal vein. Blood vessel that collects retinal venous blood drainage; exits the eye through the optic nerve.

central vision. An eye's best vision; used for reading and discriminating fine detail and color.. Results from stimulation of the fovea and the macular area.

chalazion (kuh-LAY-zee-un). Inflammed lump in a meibomian gland (in the eyelid). Inflammation usually subsides, but may need surgical removal. Sometimes called an internal hordeolum.

choroid (KOR-oyd). Vascular (major blood vessel) layer of the eye lying between the retina and the sclera. Provides nourishment to outer layers of the retina.

color blindness. Reduced ability to discriminate between colors, especially shades of red and green. Usually hereditary.

cone. Light-sensitive retinal receptor cell that provides sharp visual acuity and color discrimination.

conjunctiva (kahn-junk-TI-vuh). Transparent muccous membrane covering the outer surface of the eyeball except the cornea, and lining the inner surfaces of the eyelids.

conjunctivitis (kun-junk-tih-VI-tis), "pink eye." Inflammation of the conjunctiva. Characterized by discharge, grittiness, redness and swelling. Usually viral in origin, but may be bacterial or allergic; may be contageous.

convergence. Inward movement of both eyes toward each other, usually in an effort to maintain single binocular vision as an object approaches.

cornea (KOR-nee-uh). Transparent front part of the eye that covers the iris, pupil, and anterior chamber and provides most of an eye's optical power.

cross-eyes. See esotropia.

crystalline lens. The eye's natural lens. Transparent, biconvex intraocular tissue that helps bring rays of light to a focus on the retina.

cycloplegic refraction. Assessment of an eye's refractive error after lens accommodation has been paralyzed with cycloplegic eyedrops (to eliminate variability in optical power caused by a contracting lens).

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D

diabetic retinopathy (ret-in-AHP-uh-thee). Spectrum of retinal changes accompanying long-standing diabetes mellitus. Early stage is background retinopathy. May advance to proliferative retinopathy, which includes the growth of abnormal new blood vessels (neovascularization) and fibrous tissue.

dilated pupil. Enlarged pupil, resulting from contraction of the dilator muscle or relaxation of the iris sphincter. Occurs normally in dim illumination, or may be produced by certain drugs (mydriatics, cycloplegics) or result from blunt trauma.

diopter (D) (di-AHP-tur). Unit to designate the refractive power of a lens.

diplopia, double vision. Perception of two images from one object; images may be horizontal, vertical or diagonal.

drusen (DRU-zin). Tiny, white hyaline deposits on Bruch's membrane (of the retinal pigment epithelium). Common after age 60; sometimes an early sign of macular degeneration.

dry eye syndrome. Corneal and conjunctival dryness due to deficient tear production, predominantly in menopausal and post-menopausal women. Can cause foreign body sensation, burning eyes, filamentary keratitis, and erosion of conjunctival and corneal epithelium.

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E

ectropion (ek-TROH-pee-un). Outward turning of the upper or lower eyelid so that the lid margin does not rest against the eyeball, but falls or is pulled away. Can create corneal exposure with excessive drying, tearing, and irritation. Usually from aging.

emmetropia (em-uh-TROH-pee-uh). Refractive state of having no refractive error when accommodation is at rest. Images of distant objects are focused sharply on the retina without the need for either accommodation or corrective lenses.

entropion (en-TROH-pee-un). Inward turning of upper or lower eyelid so that the lid margin rests against and rubs the eyeball.

esotropia (ee-soh-TROH-pee-uh), cross-eyes. Eye misalignment in which one eye deviates inward (toward nose) while the other fixates normally. .

excimer laser (EKS-ih-mur). Class of ultraviolet lasers that removes tissue accurately without heating it. In refractive corneal surgery, controlled by computer to make precise pre-programmed shavings of eye tissue to produce a given optical correction. Used for photorefractive keratectomy (PRK); combined with automated lamellar keratoplasty (ALK) to produce LASIK (laser in situ keratomileusis).

exotropia (eks-oh-TROH-pee-uh), wall-eyes. Eye misalignment in which one eye deviates outward (away from nose) while the other fixates normally.

extraocular muscles (eks-truh-AHK-yu-lur). Six muscles that move the eyeball (lateral rectus, medial rectus, superior oblique, inferior oblique, superior rectus, inferior rectus).

eyelids. Structures covering the front of the eye, which protect it, limit the amount of light entering the pupil, and distribute tear film over the exposed corneal surface.

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F

farsightedness. See hyperopia.

floaters. Particles that float in the vitreous and cast shadows on the retina; seen as spots, cobwebs, spiders, etc. Occurs normally with aging or with vitreous detachment, retinal tears, or inflammation.

fluorescein angiography (FLOR-uh-seen an-jee-AH-gruh-fee). Technique used for visualizing and recording location and size of blood vessels and any eye problems affecting them; fluorescein dye is injected into an arm vein, then rapid, sequential photographs are taken of the eye as the dye circulates.

fovea (FOH-vee-uh). Central pit in the macula that produces sharpest vision. Contains a high concentration of cones and no retinal blood vessels.

fundus. Interior posterior surface of the eyeball; includes retina, optic disc, macula, posterior pole. Can be seen with an ophthalmoscope.

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G

glaucoma (glaw-KOH-muh). Group of diseases characterized by increased intraocular pressure resulting in damage to the optic nerve and retinal nerve fibers. A common cause of preventable vision loss. May be treated by prescription drugs or surgery.

gonioscopy (goh-nee-AHS-koh-pee). Examination of the anterior chamber angle through a goniolens (special type of contact lens).

H

hyperopia (hi-pur-OH-pee-uh), farsightedness. Focusing defect in which an eye is underpowered. Thus light rays coming from a distant object strike the retina before coming to sharp focus, blurring vision. Corrected with additional optical power, which may be supplied by a plus lens (spectacle or contact) or by excessive use of the eye's own focusing ability (accommodation).

hyphema (hi-FEE-muh). Blood in the anterior chamber, such as following blunt trauma to the eyeball.

I

intraocular pressure. 1. Fluid pressure inside the eye. 2. The assessment of pressure inside the eye with a tonometer. Also called tension.

IOL (intraocular lens). Plastic lens that may be surgically implanted to replace the eye's natural lens.

iris. Pigmented tissue lying behind the cornea that gives color to the eye (e.g., blue eyes) and controls amount of light entering the eye by varying the size of the pupillary opening.

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K

keratoconus (kehr-uh-toh-KOH-nus). Degenerative corneal disease affecting vision. Characterized by generalized thinning and cone-shaped protrusion of the central cornea, usually in both eyes. Hereditary.

keratometry (kehr-uh-TAH-mih-tree). Obtaining corneal curvature measurements with a keratometer.

L

lacrimal gland. Almond-shaped structure that produces tears. Located at the upper outer region of the orbit, above the eyeball.

laser. Acronym: Light Amplification by Stimulated Emission of Radiation. High energy light source that uses light emitted by the natural vibrations of atoms (of a gas or solid material) to cut, burn or dissolve tissues for various clinical purposes: in the retina, to treat diabetic retinopathy and macular degeneration, to destroy leaking and new blood vessels (neovascularization); on the iris or trabecular meshwork, to decrease pressure in glaucoma; after extracapsular cataract extraction, to open the posterior lens capsule.

LASIK (LAY-sik). Acronym: LAser in SItu Keratomileusis. Type of refractive surgery in which the cornea is reshaped to change its optical power. A disc of cornea is raised as a flap, then an excimer laser is used to reshape the intrastromal bed, producing surgical flattening of the cornea. Used for correcting myopia, hyperopia, and astigmatism.

"lazy eye." See amblyopia.

legal blindness. Best-corrected visual acuity of 20/200 or less, or reduction in visual field to 20 or less, in the better seeing eye.

lens, crystalline lens. The eye's natural lens. Transparent, biconvex intraocular tissue that helps bring rays of light to a focus on the retina.

low vision. Term usually used to indicate vision of less than 20/200.

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M

macula. Small central area of the retina surrounding the fovea; area of acute central vision.

myopia (mi-OH-pee-uh), nearsightedness. Focusing defect in which the eye is overpowered. Light rays coming from a distant object are brought to focus in front of the retina. Requires a minus lens correction to "weaken" the eye optically and permit clear distance vision.

N

nearsightedness. See myopia.

neovascularization (nee-oh-VAS-kyu-lur-ih-ZAY-shun). Abnormal formation of new blood vessels, usually in or under the retina or on the iris surface. May develop in diabetic retinopathy, blockage of the central retinal vein, or macular degeneration.

nystagmus (ni-STAG-mus). Involuntary, rhythmic side-to-side or up and down (oscillating) eye movements that are faster in one direction than the other.

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O

ophthalmologist (ahf-thal-MAH-loh-jist). Physician (MD) specializing in diagnosis and treatment of refractive, medical and surgical problems related to eye diseases and disorders.

ophthalmoscope (ahf-THAL-muh-skohp). Illuminated instrument for visualizing the interior of the eye (especially the fundus).

optic disc, optic nerve head. Ocular end of the optic nerve. Denotes the exit of retinal nerve fibers from the eye and entrance of blood vessels to the eye.

optician (ahp-TISH-un). Professional who makes and adjusts optical aids, e.g., eyeglass lenses, from refraction prescriptions supplied by an opthalmologist or optometrist.

optic nerve. Largest sensory nerve of the eye; carries impulses for sight from the retina to the brain.

optometrist (ahp-TAHM-uh-trist). Doctor of optometry (OD) specializing in vision problems, treating vision conditions with spectacles, contact lenses, low vision aids and vision therapy, and prescribing medications for certain eye diseases.

orthoptics. Discipline dealing with the diagnosis and treatment of defective eye coordination, binocular vision, and functional amblyopia by non-medical and non-surgical methods, e.g., glasses, prisms, exercises.

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P

patching. Covering an amblyopic patient's preferred eye, to improve vision in the other eye.

perimetry (puh-RIM-ih-tree). Method of charting extent of a stationary eye's field of vision with test objects of various sizes and light intensities. Aids in detection of damage to sensory visual pathways.

peripheral vision. Side vision; vision elicited by stimuli falling on retinal areas distant from the macula.

phacoemulsification (fay-koh-ee-mul-sih-fih-KAY-shun). Use of ultrasonic vibration to shatter and break up a cataract, making it easier to remove.

photophobia (foh-toh-FOH-bee-uh). Abnormal sensitivity to, and discomfort from, light. May be associated with excessive tearing. Often due to inflammation of the iris or cornea.

pinguecula (pin-GWEK-yu-luh). Yellowish-brown subconjunctival elevation composed of degenerated elastic tissue; may occur on either side of the cornea. Benign.

"pink eye." See conjunctivitis.

presbyopia (prez-bee-OH-pee-uh). Refractive condition in which there is a diminished power of accommodation arising from loss of elasticity of the crystalline lens, as occurs with aging. Usually becomes significant after age 45.

PRK (photorefractive keratectomy). Use of high intensity laser light (e.g., an excimer laser) to reshape the corneal curvature; for correcting refractive errors. Includes laser sculpting, LASIK.

progressive addition lens (PAL), progressive-power lens.. Eyeglass lens that incorporates corrections for distance vision through midrange, to near vision (usually in lower part of lens), with smooth transitions and no bifocal demarcation line.

proliterative retinopathy. See diabetic retinopathy.

pterygium (tur-IH-jee-um). Abnormal wedge-shaped growth on the bulbar conjunctiva. May gradually advance onto the cornea and require surgical removal. Probably related to sun irritation.

ptosis (TOH-sis). Drooping of upper eyelid. May be congenital or caused by paralysis or weakness of the 3rd cranial nerve or sympathetic nerves, or by excessive weight of the upper lids.

pupil. Variable-sized black circular opening in the center of the iris that regulates the amount of light that enters the eye.

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R

radial keratotomy (RK) (keh-ruh-TAH-tuh-mee). Series of spoke-like (radial) cuts made in the corneal periphery to allow the central cornea to flatten, reducing its optical power and thereby correcting nearsightedness.

refraction. Test to determine an eye's refractive error and the best corrective lenses to be prescribed. Series of lenses in graded powers are presented to determine which provide sharpest, clearest vision.

refractive error. Optical defect in an unaccommodating eye; parallel light rays are not brought to a sharp focus precisely on the retina, producing a blurred retinal image. Can be corrected by eyeglasses, contact lenses, or refractive surgery.

retina (RET-ih-nuh). Light sensitive nerve tissue in the eye that converts images from the eye's optical system into electrical impulses that are sent along the optic nerve to the brain. Forms a thin membranous lining of the rear two-thirds of the globe.

retinal detachment. Separation of the retina from the underlying pigment epithelium. Disrupts visual cell structure and thus markedly disturbs vision. Almost always caused by a retinal tear; often requires immediate surgical repair.

retinoscope (RET-in-oh-skohp). Device for measuring an eye's refractive error with no response required from the patient. Light is projected into the eye, and the movements of the light reflection from the eye are neutralized (eliminated) with lenses.

rod. Light-sensitive, specialized retinal receptor cell that works at low light levels (night vision). A normal retina contains 150 million rods.

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S

Schlemm's canal (shlemz). Circular channel deep in corneoscleral junction (limbus) that carries aqueous fluid from the anterior chamber of the eye to the bloodstream.

sclera (SKLEH-ruh). Opaque, fibrous, protective outer layer of the eye ("white of the eye") that is directly continuous with the cornea in front and with the sheath covering optic nerve behind.

secondary cataract. See after-cataract.

slit lamp. Microscope used for examining the eye; allows cornea, lens and otherwise clear fluids and membranes to be seen in layer-by-layer detail. .

Snellen chart. Test chart used for assessing visual acuity. Contains rows of letters, numbers, or symbols in standardized graded sizes, with a designated distance at which each row should be legible to a normal eye. Usually tested at 20 ft.

strabismus (struh-BIZ-mus). Eye misalignment caused by extraocular muscle imbalance: one fovea is not directed at the same object as the other.

sty, stye. Acute pustular infection of the oil glands of Zeis, located in an eyelash follicle at the eyelid margin.

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T

tonometry (tuh-NAH-mih-tree). Measurement of intraocular pressure.

trabecular meshwork (truh-BEK-yu-lur). Mesh-like structure inside the eye at the iris-scleral junction of the anterior chamber angle. Filters aqueous fluid and controls its flow into the canal of Schlemm, prior to its leaving the anterior chamber.

trifocal (TRI-foh-kul). Eyeglass lens that incorporates three lenses of different powers. The main portion is usually focused for distance (20 ft.), the center segment for about 2 ft., and the lower segment for near (14 in.).

20/20. Normal visual acuity. Upper number is the standard distance (20 feet) between an eye being tested and the eye chart; lower number indicates that a tested eye can see the same small standard-sized letters or symbols as a normal eye at 20 feet.

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U

uvea, uveal tract (YU-vee-uh). Pigmented layers of the eye (iris, ciliary body, choroid) that contain most of the intraocular blood vessels.

V

visual acuity. Assessment of the eye's ability to distinguish object details and shape, using the smallest identifiable object that can be seen at a specified distance (usually 20 ft. or 16 in.).

visual field. Full extent of the area visible to an eye that is fixating straight ahead.

vitreous (VlT-ree-us), vitreous humor. Transparent, colorless gelatinous mass that fills the rear two-thirds of the eyeball, between the lens and the retina.

vitreous detachment. Separation of vitreous gel from retinal surface. Usually innocuous, but can cause retinal tears, which may lead to retinal detachment. Frequently occurs with aging as the vitreous liquifies, or in some disease states, e.g. diabetes and high myopia.

WYZ

wall-eyes. See exotropia.

YAG laser. Laser that produces short pulsed, high energy light beam to cut perforate, or fragment tissuse.

zonules (ZAHN-yoolz). Anatomy. Radially arranged fibers that suspend the lens from the ciliary body and hold it in position. x

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