Please enable JS

Eye Library / Glossary

Accommodation: The automatic adjustment of the eye for seeing at different distances, effected chiefly by changes in the convexity of the crystalline lens

Ametropia: A refractive error (such as myopia, hyperopia, or astigmatism) in which images fail to focus upon the retina, requiring spectacle lenses or contact lenses to refocus the incoming light onto the retina

Aniridia: Congenital or traumatically induced absence or defect of the iris 

Anisocoria: Inequality in the size of the pupils of the eyes, often from injury or disease

Anisometropia: A difference in refractive error between the two eyes 

Anterior chamber: The space in the eye bounded in front by the cornea and in back by the iris and middle part of the lens; contains the aqueous humor

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.

Antioxidant: Substance that inhibits oxidation and can guard the body from the 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. It has been theorized that antioxidants can help prevent macular degeneration and other serious eye diseases; many studies are being conducted in this area.

Aphakia: Absence of the eye's crystalline lens, such as after cataract extraction.

Aqueous humor: The transparent fluid occupying the anterior compartment (the space between the cornea and the crystalline lens) of the eye; produced by ciliary epithelium and circulates into the posterior chamber (between the iris and the crystalline lens), through the pupil, into the anterior chamber (between the cornea and the iris), and out of the eye through the trabecular meshwork and canal of Schlemm; nourishes the lens and epithelial cells

Arcus senilis: A harmless opaque, cloudy ring or arc in the corneal periphery, most often seen in aged persons due to the deposition of fat granules in the cornea or to hyaline degeneration; may indicate elevated blood cholesterol levels

Argon laser: An ion laser, using argon as the active medium, with two dominant wavelengths—514 nm “green” and 488 nm “blue”—used in argon laser trabeculoplasty (ALT) in which the fluid channels of the trabecular meshwork of the eye are opened, helping aqueous fluid to drain out of the eye more easily and reducing intraocular pressure to help control glaucoma; also used to treat neovascularization in diabetic retinopathy, where the hemoglobin in blood cells absorbs the laser light, causing heat damage and sealing of the blood vessels and controlling further bleeding into the vitreous

Artificial tears: Used to soothe the eyes, moisturize dry spots on the eyes, supplement the eye’s own tears, and protect the eye from irritation. They are often formulated to match the pH of human tears

Astigmatism: A defect of an ocular structure (most commonly the cornea or the crystalline lens) causing rays from a point to fail to meet in a single focal point, resulting in an imperfect blurred or smeared image; a defect of vision due to astigmatism of the refractive system of the eye

Binocular vision: The ability of the visual cortex (located in the occipital lobe of the brain) to combine the impulses it receives from the images on the retinas of both eyes into one single image

Bipolar cells: Intermediary nerve cells in the retina which transmit visual signals from the photoreceptor cells to the ganglion cells

Blind spot: Small area in the visual field corresponding to the retina’s optic disc or optic nerve head (where the optic nerve emerges), where no photoreceptors are present and where there is no sensitivity to light; not noticed with both eyes open because the part of the visual field containing the blind spot of one eye is overlapped by a light-sensitive area in the other eye

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.

Bowman’s membrane: The thin outer layer of the substantia propria of the cornea, immediately underlying the epithelium

Bruch’s membrane: The membrane separating the pigment epithelium of the retina and the choriocapilaris of the choroid

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.

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 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: Inflammed lump in a meibomian gland (in the eyelid). Inflammation usually subsides, but may need surgical removal. Sometimes called an internal hordeolum.

Chemosis: Conjunctival swelling that is often caused by an allergy.

Choroid: Vascular layer of the eye lying between the retina and the sclera. Provides nourishment to outer layers of the retina.

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.

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. 
Conjunctiva: Transparent muccous membrane covering the outer surface of the eyeball except the cornea, and lining the inner surfaces of the eyelids.

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

Cornea: Transparent front part of the eye that covers the iris, pupil, and anterior chamber and provides most of an eye's optical power.

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 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, Fuch's dystrophy and lattice dystrophy. Symptoms include blurred vision, foreign body sensation, light sensitivity, eye pain or discomfort and vision loss.

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 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.

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).

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

Descemet's membrane: Corneal layer between the stroma and the endothelium

Diabetic retinopathy: 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): unit to designate the refractive power of a lens

Diplopia:  It is a double vision; perception of two images from one object; images may be horizontal, vertical or diagonal

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

Dk/t Dk: 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

Drusen: 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

Ectropion:  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: 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

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

Entropion: Inward turning of upper or lower eyelid so that the lid margin rests against and rubs the eyeball

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

Esotropia: Eye misalignment in which one eye deviates inward (toward nose) while the other fixates normally

Excimer laser:  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: Eye misalignment in which one eye deviates outward (away from nose) while the other fixates normally

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."

Extraocular muscles: 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.

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: 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: 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

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: 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.

Glaucoma: 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: Examination of the anterior chamber angle through a goniolens (special type of contact lens).

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.

Hyperopia:  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: Blood in the anterior chamber, such as following blunt trauma to the eyeball.

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.

Iritis: Inflammation of the iris.

Keratitis: nflammation of the cornea, caused by an infection or inflammatory process. Symptoms include eye pain or discomfort, light sensitivity, foreign body sensation, grittiness and tearing.

Keratoconjunctivitis: Inflammation of the cornea and conjunctiva.
keratoconjunctivitis sicca Also called dry eye syndrome. Chronic lack of sufficient lubrication and moisture in the eye.

Keratoconus: Degenerative corneal disease affecting vision. Characterized by generalized thinning and cone-shaped protrusion of the central cornea, usually in both eyes. Hereditary.

Keratometry: Obtaining corneal curvature measurements with a keratometer.

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

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  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.

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

Lens dislocation: Full or partial displacement of the eye's lens. Dislocation is often caused by trauma to the eye or head, but may also be inherited or come as the result of certain systemic conditions, such as Marfan's syndrome or homocystinuria. Blurred vision is a typical symptom, and some people may experience double vision and/or develop glaucoma.

Leukocoria: White pupil. Causes include congenital cataract, retinoblastoma, intraocular infection, Coat's disease and retinopathy of prematurity.

Limbus: Boundary area connecting the cornea and sclera; the three form the eye's outermost layer
low vision  Term usually used to indicate vision of less than 20/200. lutein An antioxidant that is found throughout the body, but is concentrated in the macula.

Lutein: is believed to help protect the eyes from free radical damage caused by the sun's harmful rays.

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

Microphthalmia: 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.

Myopia: 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.

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

Ophthalmologist: Physician (MD) specializing in diagnosis and treatment of refractive, medical and surgical problems related to eye diseases and disorders

Ophthalmoscope: Illuminated instrument for visualizing the interior of the eye (especially the fundus)

Optic disc: 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: Professional who makes and adjusts optical aids, e.g., eyeglass lenses, from refraction prescriptions supplied by an ophthalmologist.

Optic nevre:  Largest sensory nerve of the eye; carries impulses for sight from the retina to the brain


Patching: Covering an amblyopic patient's preferred eye, to improve vision in the other eye.
perimetry  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:  Use of ultrasonic vibration to shatter and break up a cataract, making it easier to remove

Photophobia: Abnormal sensitivity to, and discomfort from, light. May be associated with excessive tearing. Often due to inflammation of the iris or cornea

Pinguecula: Yellowish-brown subconjunctival elevation composed of degenerated elastic tissue; may occur on either side of the cornea. It is a benign lesion 

Presbyopia:  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: use of high intensity laser light (e.g., an excimer laser) to reshape the corneal curvature; for correcting refractive errors

Progressive 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

Pterygium:  abnormal wedge-shaped growth on the bulbar conjunctiva. May gradually advance onto the cornea and require surgical removal. Probably related to sun irritation

Ptosis: 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

Punctal plugs: tiny inserts often made of plastic that are placed in channels or ducts of the eye where moisture drainage occurs.They can help stop excessive drainage to keep the eye moistened in conditions such as dry eye syndrome. For more information, please see our dry eye syndrome article.

Pupil: variable-sized black circular opening in the center of the iris that regulates the amount of light that enters the eye.

Radial keratotomy (RK):  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:  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: 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.

Schlemm canal: circular channel deep in corneoscleral junction (limbus) that carries aqueous fluid from the anterior chamber of the eye to the bloodstream

Sclera: The outer coat of the eyeball that forms the visible white of the eye and surrounds the optic nerve at the back of the eyeball

Scleritis: Inflammation of the sclera. Autoimmune disorders are the most common cause. Symptoms include a red or pink eye, eye pain, light sensitivity, tearing and blurred vision.

Scotoma: Blind spot within the field of view

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

Sjogren's syndrome: An inflammatory autoimmune disorder characterized by a dry mouth and dry eyes. Additional eye symptoms include burning, discharge, foreign body sensation, itching and light sensitivity

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:  Eye misalignment caused by extraocular muscle imbalance: one fovea is not directed at the same object as the other

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

Tonometry:  Measurement of intraocular pressure.

Trabecular meshwork:  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 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.).

Uvea: pigmented layers of the eye (iris, ciliary body, choroid) that contain most of the intraocular blood vessels.

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 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.

Xanthelasma:  Yellow, fatty spot or bump on the inner corner of either the upper eyelid, the lower one or both eyelids, often caused by a lipid disorder such as high cholesterol.

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

Zonules: Radially arranged fibers that suspend the lens from the ciliary body and hold it in position