Accommodation: The adjustment of the eye for seeing at near distances, accomplished by changing the shape of the lens through action of the ciliary muscle, thus focusing a clear image on the retina.

Agnosia: Inability to recognize common objects despite an intact visual apparatus.

Albinism: A hereditary deficiency of melanin pigment in the retinal pigment epithelium, iris, and choroid.

Amaurosis fugax: Transient loss of vision.

Amblyopia: Reduced visual acuity (uncorrectable with lenses) in the absence of detectable anatomic defect in the eye or visual pathways.

Ametropia: See Refractive error.

Amsler grid: A chart with vertical and horizontal lines used for testing the central visual field.

Angiography: A diagnostic test in which the vascular system is examined. The ocular circulation can be highlighted by intravenous injection of either fluorescein, which particularly demonstrates the retinal circulation, or indocyanine green, to demonstrate the choroidal circulation.

Aniridia: Congenital absence of the iris.

Aniseikonia: A condition in which the image seen by one eye differs in size or shape from that seen by the other.

Anisocoria: Unequal pupillary size.

Anisometropia: Difference in refractive error of the eyes.

Anophthalmos: Absence of a true eyeball.

Anterior chamber: Space filled with aqueous bounded anteriorly by the cornea and posteriorly by the iris.

Aphakia: Absence of the crystalline lens.

Aqueous: Clear, watery fluid that fills the anterior and posterior chambers.

Asthenopia: Eye fatigue from muscular, environmental, or psychologic causes.

Astigmatism: Refractive error that prevents the light rays from coming to a point focus on the retina because of different degrees of refraction in the various meridians of the cornea or crystalline lens.

Axis: The meridian specifying the orientation of a cylindric lens.

Binocular vision: Ability of the eyes to focus on one object and then to fuse two images into one.

Biomicroscope: See Slitlamp.

Bitot's spots: Keratinization of the bulbar conjunctiva near the limbus, resulting in a raised spot-a feature of vitamin A deficiency.

Blepharitis: Inflammation of the eyelids.

Blepharoptosis: Drooping of the eyelid.

Blepharospasm: Involuntary spasm of the lids.

Blind spot: "Blank" area in the visual field, corresponding to the light rays that come to a focus on the optic nerve.

Blindness: In the USA, the usual definition of blindness is corrected visual acuity of 20/200 or less in the better eye, or a visual field of no more than 20 degrees in the better eye.

Botulinum toxin: Neurotoxin A of the bacterium Clostridium botulinum used in very small doses to produce temporary paralysis of the extraocular or facial muscles.

Buphthalmos: Large eyeball in infantile glaucoma.

Canal of Schlemm: A circular modified venous structure in the anterior chamber angle that drains aqueous to the aqueous veins.

Canaliculus: Small tear drainage tube in inner aspect of upper and lower lids leading from the punctum to the common canaliculus and then to the tear sac.

Canthotomy: Usually implies lateral canthotomy-cutting of the lateral canthal tendon for the purpose of widening the palpebral fissure.

Canthus: The angle at either end of the eyelid aperture; specified as outer and inner.

Cataract: An opacity of the crystalline lens.

Cataract extraction: Removal of a cataract, either by removal of the lens complete with its capsule (intracapsular cataract extraction), or by removal of the lens contents after opening the capsule (extracapsular cataract extraction).

Chalazion: Granulomatous inflammation of a meibomian gland.

Chemosis: Conjunctival edema.

Choroid: The vascular middle coat between the retina and sclera.

Ciliary body: Portion of the uveal tract between the iris and the choroid. It consists of ciliary processes and the ciliary muscle.

Coloboma: Congenital cleft due to the failure of some portion of the eye or ocular adnexa to complete growth.

Color blindness: Diminished ability to perceive differences in color.

Concave lens: Lens having the power to diverge rays of light; also known as diverging, reducing, negative, or minus lens, denoted by the sign (-), used to correct myopia.

Cones and rods: Two kinds of retinal receptor cells. Cones are concerned with visual acuity and color discrimination; rods, with peripheral vision under decreased illumination.

Conjunctiva: Mucous membrane that lines the posterior aspect of the eyelids and covers the anterior sclera.

Convergence: The process of directing the visual axes of the eyes to a near point.

Convex lens: Lens having power to converge rays of light and to bring them to a focus; also known as converging, magnifying, or plus lens, denoted by the sign (+), used to correct hyperopia or presbyopia.

Cornea: Transparent portion of the outer coat of the eyeball forming the anterior wall of the anterior chamber.

Corneal contact lenses: Thin lenses that fit directly on the cornea.

Corneal graft (keratoplasty): Operation to restore vision by replacing a section of opaque cornea with transparent cornea, either involving the full thickness of the cornea (penetrating keratoplasty) or only a superficial layer (lamellar keratoplasty). The donor cornea may be from the same human (autograft), another human (homograft), or another species (heterograft).

Cover test A method of determining the presence and degree of phoria or tropia by covering one eye with an opaque object, thus eliminating fusion.

Cross cylinder: A specialized spherocylindrical lens used to measure astigmatism.

Crystalline lens: A transparent biconvex structure suspended in the eyeball between the aqueous and the vitreous. Its function is to bring rays of light to a focus on the retina. Accommodation is produced by variations in the magnitude of this effect. (Now usually called simply the lens.)

Cyclodestructive procedures: Surgical techniques to reduce aqueous production by destroying portions of the ciliary body in the treatment of intractable glaucoma, using cryotherapy (cyclocryotherapy), lasers (cyclophotocoagulation), or diathermy.

Cycloplegic: A drug that temporarily puts the ciliary muscle at rest, paralyzing accommodation.

Cylindrical lens: A segment of a cylinder the refractive power of which varies in different meridians, used to correct astigmatism.

Dacryocystitis: Infection of the lacrimal sac.

Dacryocystorhinostomy: A procedure by which a communication is made between the nasolacrimal duct and the nasal cavity to relieve an obstruction in the nasolacrimal duct, or sac.

Dark adaptation. The ability to adjust to decreased illumination.

Diopter: Unit of measurement of refractive power of lenses.

Diplopia: Seeing one object as two.

"E" test: A system of testing visual acuity in illiterates, particularly preschool children.

Ectropion: Turning out of the eyelid.

Emmetropia: Absence of refractive error.

Endolaser: Application of laser from a probe inserted into the globe.

Endophthalmitis: Extensive intraocular infection.

Enophthalmos: Abnormal retrodisplacement of the eyeball.

Entropion: A turning inward of the eyelid.

Enucleation: Complete surgical removal of the eyeball.

Epicanthus: Congenital skin fold that overlies the inner canthus.

Epiphora: Tearing.

Esophoria: A tendency of the eyes to be convergent.

Esotropia: A manifest inward deviation of one eye.

Evisceration: Removal of the contents of the eyeball.

Exenteration: Removal of the entire contents of the orbit, including the eyeball and lids.

Exophoria: A tendency of the eyes to be divergent.

Exophthalmos: Abnormal protrusion of the eyeball.

Exotropia: A manifest outward deviation of one eye.

Far point: The point at which the eye is focused when accommodation is completely relaxed.

Farsightedness: See Hyperopia.

Field of vision: The entire area that can be seen without shifting the gaze.

Floaters: Moving images in the visual field due to vitreous opacities.

Focus: A point to which rays of light are brought together to form an image; focal distance is the distance between a lens and its focal point.

Fornix: The junction of the palpebral and bulbar conjunctiva.

Fovea: Depression in the macula adapted for most acute vision.

Fundus: The posterior portion of the eye visible through an ophthalmoscope.

Fusion: Coordinating the images received by the two eyes into one image.

Glaucoma: Disease characterized by abnormally increased intraocular pressure, optic atrophy, and loss of visual field.

Gonioscopy: A technique of examining the anterior chamber angle, utilizing a corneal contact lens, magnifying device, and light source.

Hemianopia: Blindness in one-half of the field of vision of one or both eyes.

Heterophoria (phoria): A tendency of the eyes to be misaligned.

Heterotropia: See Strabismus.

Hippus: Exaggerated spontaneous rhythmic movements of the iris.

Hordeolum, external (sty): Infection of the glands of Moll or Zeis.

Hordeolum, internal: Meibomian gland infection.

Hyperopia, hypermetropia (farsightedness): A refractive error in which the focus of light rays from a distant object is behind the retina.

Hyperphoria: A tendency of one eye to deviate upward.

Hypertropia: A manifest upward deviation of one eye in relation to the other.

Hyphema: Blood in the anterior chamber.

Hypopyon: Pus in the anterior chamber.

Hypotony: Abnormally soft eye from any cause.

Injection: Congestion of blood vessels.

Iris: Colored, annular membrane, suspended behind the cornea and immediately in front of the lens.

Ishihara color plates: A test for color vision based on the ability to see patterns in a series of multicolored charts.

Isopter: An object for testing visual fields. Isopters can be of different colors and sizes so as to differentiate relative visual field defects from absolute defects.

Jaeger test: A test for near vision using lines of various sizes of type.

Keratic precipitate (KP): Accumulation of inflammatory cells on the posterior cornea in uveitis.

Keratitis: Inflammation of the cornea.

Keratoconus: Cone-shaped deformity of the cornea.

Keratomalacia: Corneal softening, usually associated with avitaminosis A.

Keratometer: An instrument for measuring the curvature of the cornea, used in fitting contact lenses.

Keratopathy, bullous: Swelling of the cornea with painful blisters in the epithelium due to excessive corneal hydration.

Keratoplasty: See Corneal graft.

Keratoprosthesis: Plastic implant surgically placed in an opaque cornea to achieve an area of optical clarity.

Keratotomy: An incision in the cornea. Radial keratotomy is a procedure in which radial incisions are made in the cornea to correct myopia.

Koeppe nodule: Accumulation of inflammatory cells on the iris in uveitis.

Lacrimal sac: The dilated area at the junction of the nasolacrimal duct and the canaliculi.

Lens: A refractive medium having one or both surfaces curved. (See also Crystalline lens.)

Limbus: Junction of the cornea and sclera.

Macula lutea: The small avascular area of the retina surrounding the fovea, containing yellow xanthophyll pigment.

Maddox rod: A red lens composed of parallel series of strong cylinders through which a point of light is viewed as a red line-used to measure phorias.

Magnification: The ratio of the size of an image to the size of its object.

Megalocornea: Abnormally large cornea (gt; 13 mm in diameter).

Metamorphopsia: Wavy distortion of vision.

Microphthalmos: Abnormally small eye with abnormal function (see Nanophthalmos).

Miotic: A drug causing pupillary constriction.

Mydriatic: A drug causing pupillary dilation.

Myopia (nearsightedness): A refractive error in which the focus for light rays from a distant object is anterior to the retina.

Nanophthalmos: Abnormally small eye with normal function (see Microphthalmos).

Near point: The point at which the eye is focused when accommodation is fully active.

Nearsightedness: See Myopia.

Nystagmus: An involuntary oscillation of the eyeball that may be horizontal, vertical, torsional, or mixed.

Ophthalmia neonatorum: Conjunctivitis in the newborn.

Ophthalmoscope: An instrument with a special illumination system for viewing the inner eye, particularly the retina and associated structures.

Optic atrophy: Optic nerve degeneration.

Optic disk: Ophthalmoscopically visible portion of the optic nerve.

Optic nerve: The nerve that carries visual impulses from the retina to the brain.

Orbital cellulitis: Inflammation of the tissues surrounding the eye.

Orthoptics: The study and treatment of defects of binocular visual function or of the muscles controlling movement of the eyeballs.

Oscillopsia: The subjective illusion of movement of objects that occurs with some types of nystagmus.

Palpebral: Pertaining to the eyelid.

Pannus: Infiltration of the cornea with blood vessels.

Panophthalmitis: Inflammation of the entire eyeball.

Papilledema: Swelling of the optic disk due to raised intracranial pressure.

Papillitis: Optic nerve head inflammation.

Partially seeing child: For educational purposes, a partially seeing child is one who has a corrected visual acuity of 20/70 or less in the better eye.

Perimeter: An instrument for measuring the field of vision.

Peripheral vision: Ability to perceive the presence, motion, or color of objects outside of the direct line of vision.

Phacoemulsification and phacofragmentation: Techniques of extracapsular cataract extraction in which the nucleus of the lens is disrupted into small fragments by ultrasonic vibrations, thus allowing aspiration of all the lens matter through a small wound.

Phakomatoses: A group of hereditary diseases characterized by the presence of spots, cysts, and tumors in various parts of the body-eg, neurofibromatosis, Von Hippel-Lindau disease, tuberous sclerosis.

Phlyctenule: Localized lymphocytic infiltration of the conjunctiva.

Phoria: See Heterophoria.

Photocoagulation: Thermal damage to tissues due to absorption of high levels of light (including laser) energy.

Photodecomposition: Tissue damage by direct separation of chemical bonds by absorption of very short wavelength ultraviolet light (eg, from excimer lasers).

Photodisruption: Tissue damage produced by the breakdown of "plasma," which is a state of ionization created by spot focusing a high-energy laser source (eg, neodymium:YAG).

Photophobia: Abnormal sensitivity to light.

Photopsia: Appearance of sparks or flashes within the eye due to retinal irritation.

Phthisis bulbi: Atrophy of the eyeball with blindness and decreased intraocular pressure, due to end-stage intraocular disease.

Placido's disk: A disk with concentric rings used to determine the regularity of the cornea by observing the ring's reflection on the corneal surface.

Poliosis: Depigmentation of the eyelashes.

Posterior chamber: Space filled with aqueous anterior to the lens and posterior to the iris.

Presbyopia ("old sight"): Physiologically blurred near vision, commonly evident soon after age 40, due to reduction in the power of accommodation.

Prism: A wedge of transparent material that deviates light rays without changing their focus.

Prism diopter: The unit of prism power.

Pseudoisochromatic charts: Charts with colored dots of various hues and shades forming numbers, letters, or patterns, used for testing color discrimination.

Pseudophakia: Presence of an artificial intraocular lens implant following cataract extraction.

Pterygium: A triangular growth of tissue that extends from the conjunctiva over the cornea.

Ptosis: Drooping of the eyelid.

Puncta: External orifices of the upper and lower canaliculi.

Pupil: The round hole in the center of the iris that corresponds to the lens aperture in a camera.

Refraction: (1) Deviation in the course of rays of light in passing from one transparent medium into another of different density. (2) Determination of refractive errors of the eye and correction by lenses.

Refractive error (ametropia): An optical defect that prevents light rays from being brought to a single focus on the retina.

Refractive index: The ratio of the speed of light in a vacuum to the speed of light in a given material.

Refractive keratoplasty: Surgery of the cornea to correct refractive errors.

Refractive media: The transparent parts of the eye having refractive power.

Retina: Innermost coat of the eye, consisting of the sensory retina, which is composed of light-sensitive neural elements connecting to other neural cells, and the retinal pigment epithelium.

Retinal detachment: A separation of the neurosensory retina from the pigment epithelium and choroid.

Retinitis pigmentosa: A hereditary degeneration of the retina.

Retinoscope: An instrument specially designed for refracting an eye objectively.

Rods: See Cones and rods.

Sclera: The white part of the eye-a tough covering that, with the cornea, forms the external protective coat of the eye.

Scleral spur: The protrusion of sclera into the anterior chamber angle.

Scotoma: A blind or partially blind area in the visual field.

Slitlamp: A combination light and microscope for examination of the eye.

Snellen chart: Used for testing central visual acuity. It consists of lines of letters or numbers, in graded sizes drawn to Snellen measurements.

Sphincterotomy: A surgical incision of the iris sphincter muscle.

Staphyloma: A thinned part of the coat of the eye, causing protrusion.

Strabismus (heterotropia, tropia): Misalignment of the eyes.

Sty: See Hordeolum, external.

Symblepharon: Adhesions between the bulbar and palpebral conjunctiva.

Sympathetic ophthalmia: Inflammation in both eyes following trauma.

Synechia: Adhesion of the iris to the cornea (anterior synechia) or lens (posterior synechia).

Syneresis: A degenerative process within a gel, involving a drawing together of particles of the dispersed medium, separation of the medium, and shrinkage of the gel. Specifically applied to the vitreous.

Tarsorrhaphy: A surgical procedure by which the upper and lower lid margins are united.

Tonometer: An instrument for measuring intraocular pressure.

Trabeculectomy: Surgical procedure for creating an additional aqueous drainage channel in the treatment of glaucoma.

Trabeculoplasty: Laser photocoagulation of the trabecular meshwork in the treatment of open-angle glaucoma.

Trachoma: A serious form of infectious keratoconjunctivitis.

Trichiasis: Inversion and rubbing of the eyelashes against the globe.

Tropia: See Strabismus.

Uvea (uveal tract): The iris, ciliary body, and choroid.

Uveitis: Inflammation of one or all portions of the uveal tract.

Visual acuity: Measure of the optical resolution of the eye.

Visual axis: An imaginary line that connects a point in space (point of fixation) with the fovea centralis.

Vitiligo: Localized patchy decrease or absence of pigment on the skin.

Vitreous: Transparent, colorless mass of soft, gelatinous material filling the eyeball behind the crystalline lens.

Xerosis: Drying of tissues lining the anterior surface of the eye.

Zonule: The numerous fine tissue strands that stretch from the ciliary processes to the crystalline lens equator (360 degrees) and hold the lens in place.

Zonulolysis: Lysis of the zonule, as with chymotrypsin, to facilitate removal of the lens in intracapsular cataract surgery.