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Vision Glossary

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A Back to Top Abacus: Instrument with sliding counters used for counting or performing calculations.
Accommodation: Ability of the eye to focus at different distances.
Achromatopsia: Congenital defect in which the receptor cells on the retina, called cones, are either absent or defective, resulting in the inability to see colors.
Acuity Loss: Vision problem that prevents an individual from seeing clearly such as cataracts, dystrophy, nystagmus, and retinitis pigmentosa.
Albinism: Partial or total absence of pigment in the skin, hair, and eyes; frequently accompanied by strabismus, nystagmus, reduced acuity, light sensitivity, and poor depth perception.
Amblyopia (Lazy Eye): Reduction in or dimness of vision; no detectable organic cause.
Aniridia: Congenital absence of all or part of the iris.
Anophthalmia: Congenital absence of one or both eyes.
Anterior Chamber: Located behind the cornea; filled with aqueous humor.
Aphakia: Absence of the crystalline lens of the eye, which may be congenital or surgically caused. Causes a loss of accommodation and hyperopia.
Aqueous Humor: Clear liquid that fills the anterior chamber and supplies oxygen and nutrients to the inner part of the eye.
Astigmatism: Abnormal condition of the eye in which the spherical curve of the cornea is irregular, causing blurred vision; may be corrected with contact lenses or eyeglasses.
Back to Top B Back to Top Binocular Vision: Ability to use both eyes simultaneously to focus on the same object, fusing the two images into one.
Blind Spot: Blank area in the visual field corresponding to the position of the optic nerve, or an area of absent or reduced sensitivity anywhere in the visual field.
Blindness: Inability to see; absence or severe reduction of vision.
Braille: Raised dots that represent letters and numbers; used by blind individuals.
Brailler: Machine that creates braille.
C Back to Top Cataract: Condition in which the lens of the eye appears cloudy or opaque.
Central Field Loss: Eye condition in which affected individuals cans see well from the sides but have poor or no central vision; the opposite of tunnel vision.
Choroid: Located behind the retina; filled with blood vessels that provide oxygen and nutrients to keep the retina alive.
Choroideremia: Condition in which vision is impacted first with the onset of night blindness, then field restrictions, and then acuity and color vision, resulting in blindness in later years; occurs more in males.
Ciliary Bodies: Muscles attached to the iris that pull on the eye lens to change its shape and help to focus the vision. Also produces aqueous humor, the clear fluid that fills the front of the eye.
Closed Circuit Television (CCTV): High tech vision device; electronically enlarges print material on a television screen.
Coloboma: Notch or cleft in the pupil, iris, lens, retina, choroid, or optic nerve. Its location determines the degree of vision loss. There may be decreased acuity, photophobia, field loss, refractive errors, and poor depth perception.
Color Deficiency: (Color blindness) Inability, partial or complete, to discriminate among colors.
Cone: Part of the photoreceptors that gives information about color. There are red, green, and blue cones, which each respond to different frequencies of the light spectrum. The retina contains approximately six million cones.
Conjunctiva: Mucous membrane that lines the eyelids and covers the front part of the eyeball.
Conjunctivitis: Pink eye or inflammation of the conjunctiva; caused by bacteria, viruses, or allergies.
Cornea: The transparent dome-shaped window covering the eye where light enters the eyeball.
Cortical Visual Impairment (CVI): The brain is unable to process the visual information sent to it from the eyes through the visual pathways.
D Back to Top Delayed Visual Maturation (DVM): Absence of visual responsiveness despite normal and intact visual pathways. In time, it is followed by a gradual improvement of visual functioning when the child is two to three years old.
Depth Perception: Ability to judge depth or the relative distance of objects in space and to orient one's position in relation to them.
Diabetic Retinopathy: Disorder in which repeated hemorrhage results in permanent opacity of the vitreous humor; blindness may eventually result; occurs most frequently in individuals with long-standing, poorly controlled diabetes.
Diopter: Unit of measurement used to indicate how much light rays are refracted or bent by a lens.
Diplopia: Seeing one object as if it were two.
Dome Magnifier: Low tech vision device.
E Back to Top Esotropia: Abnormal turning inward of one eye.
Exotropia: Abnormal turning outward of one eye.
Extraocular Muscles: Six muscles which control eye movement: superior oblique, superior rectus, lateral rectus, inferior oblique, inferior rectus, and medial rectus.
F Back to Top Field Loss: Reduction in the area visible to the eyes when they are fixating straight ahead.
Fixation Ability: Ability of the eyes to direct gaze on an object and hold it steadily in view.
Focus: Point to which rays of light are brought to a sharp image point after passing through a lens.
Fovea: Small depression on the retina of the eye; the part of the macula adapted for the clearest vision.
Functional Vision: Presence of enough usable vision that the individual has the ability to use sight as a primary channel for learning or living. Relates to the total act of seeing and how the individual uses sight to function.
Functional Vision Assessment: Testing how an individual uses vision.
G Back to Top Glaucoma: Disease of the eye caused by increased intraocular pressure, resulting either from a malformation or a malfunction of the eye's drainage structrues.
H Back to Top Hemianopsia: Field loss that occurs when a specific portion of the optic pathway malfunctions, usually because of pressure from a tumor. The degree of the field loss depends on the type, amount, and location of the pressure. Field losses can be corresponding (the same in both eyes) or opposite. They may involve half fields or quadrants, or affect upper or lower fields..
Hydrocephalus: Condition in which dilated cerebral ventricles inhibit the normal flow of cerebrospinal fluid (CSF). The CSF accumulates in the skull and puts pressure on the brain tissue, which includes the optic nerve and visual cortex. Visual functioning varies. Eye conditions include strabismus, nystagmus, refractive errors, optic nerve atrophy, and DVM.
Hyperopia: Farsightedness; condition of the eye in which light rays from distance objects are brought to focus behind the retina when the eye is at rest, making it difficult to see up close.
I Back to Top Inferior Oblique: One of four muscles that controls the vertical motion of the eyeball. Rotates the eye so that the top of the eye moves away from the nose.
Inferior Rectus: One of four muscles that controls the vertical motion of the eyeball. Moves the eye down.
Iris: Ring-shaped muscle that controls the size of the pupil and the amount of light that enters the eye.
K Back to Top Keratoconus: Rare degenerative disease, occurring in the second decade of life, in which the cornea becomes cone-shaped, thins in the center, and may rupture.
L Back to Top Lateral Rectus: One of two muscles that controls the horizontal motion of the eyeball. Moves the eye away from the nose.
Leber Congenital Amaurosis: Rare genetic eye disorder. Onset occurs at birth or within the first few years of life. Symptoms may include strabismus, nystagmus, unusual sensitivity to light, clouding of the lenses of the eye, protrusion of the front of the eye, hearing loss, mental retardation, and delay of mental and muscular activity.
Leber's Optic Atrophy: Rare hereditary condition, largely affecting males, in which vision loss of rapid onset occurs. It starts in one eye, moves to the other eye, and often leads to total blindness.
Legally Blind: 20/200 or worse in the better eye with the best possible correction.
Lens: Clear, flexible disk that helps the eye focus light onto the retina.
Light Perception: Ability to distinguish the absence or presence of light.
Low Vision Aids: Optical and non-optical devices of various types, such as magnifiers, lenses, hand-held telescopes, and reading stands, which are useful to people with visual impairments.
Low Vision Specialist: Professional trained to help individuals enhance and utilize their residual vision using various optical devices.
M Back to Top Macula: Small area of the retina of the eye that surrounds the fovea and that, with the fovea, is responsible for the detailed central vision.
Macular Area: Section of the retina where cones are very dense. (These cones discriminate fine details straight ahead.)
Macular Degeneration: Hereditary condition in which the cones in the central part of the retina degenerate.
Medial Rectus: One of two muscles that controls the horizontal motion of the eyeball. Moves the eye towards the nose.
Meningitis: Inflammation of the lining surrounding the brain and spinal cord, which may result in loss of sight or changes in vision.
Microphthalmia: Hereditary, congenital birth defect in which one or both eyes are much smaller than normal.
Microscope: Low tech vision device that enlarges objects.
Monocular: Pertaining to one eye. Also is used to mean a hand-held telescope (low tech device).
Mowat Sensors: Devices that are attached to travel canes to translate objects in space into specific vibration patterns.
Muscle Balance: Ability of the six extraocular eye muscles of each eye to pull together to allow binocular vision to occur in all directions.
Myopia: Defect in vision resulting in the inability to see objects distinctly at a distance.
N Back to Top Nearsighted: Able to see up-close better than far away.
Neural Impulses: Light that has been transformed into electric signals that travels along the optic nerve to the brain.
Night Blindness: An individual with this condition may be able to see with no problems by day or in good illumination (light), but is unable to see at night or in faint light. Can be caused by any eye condition that affects the rods of the eye.
Nonprogressive Vision Problem: Vision problem that remains the same over time (e.g., does not improve or worsen).
Nystagmus: Involuntary rapid movements of the eyes.
O Back to Top Ocular: Pertaining to the eye.
Ocularist: Professional who specializes in the making and fitting of artificial eyes.
Oculomotor Apraxia: Rare inherited neurological disorder present at birth which is characterized by impairment in horizontal eye movements and difficulty converging (to move toward one point) and diverging (to move outward from a common point) on an object.
Oculomotor Problems: Individuals with these problems have trouble moving their eyes, so it is difficult for them to look at and follow the movement of objects and people.
Ophthalmologist: Doctor who routinely examines eyes for infections, injury, and malformations; also tests for visual acuity and problems with visual field, color, and contrast sensitivity.
Optic Disc: The area from where all of the neural fibers leave the eyeball.
Optic Nerve: Nerve that transmits neural impulses from the retina to the brain.
Optic Nerve Atrophy: Inability of the optic nerve to conduct electrical impulses to the brain, resulting in loss of vision, which ranges from minimal to total. It is caused by pressure on the optic nerve from glaucoma, tumors, hydrocephalus, trauma, etc.
Optic Nerve Hypoplasia: Failure of the optic nerve(s) to develop normally, such that the nerve(s) are not fully functioning.
Optician: Professional who grinds lenses, fits them into frames, and dispenses and adjusts glasses or other optical devices on the written prescription of an optometrist or physician.
Optometrist: Licensed, nonmedical eye practitioner who treats vision problems nonmedically through use of glasses, prisms, and eye exercises. In some states, he/she may also diagnose and treat some eye conditions with medications.
Orientation and Mobility: Orientation is knowing where you are and being able to plan how to go where you want to go. Mobility is the actual movement from place to place.
Orientation and Mobility Specialist: Professional who has specialized training to teach children with visual impairments; teaches about space, directions, travel techniques, and adaptive devices.
Orthopist: Provides vision training to help the eyes work together in order to produce binocular vision.
P Back to Top Peripheral Vision: Outer part of the visual field; side vision.
Persistent Hyperplastic Primary Vitreous (PHPV): Condition that usually affects one eye, which is slightly small and has a cataract. Surgery is often needed to salvage vision, and visual prognosis ranges from mild to severe.
Photons: Little bits of energy that make up light.
Photophobia: Abnormal sensitivity to or discomfort from light.
Photoreceptors: Specialized cells called rods and cones that make up the retina; triggered by light that enters the eyes.
Posterior Chamber: Interior area in the eye behind the lens.
Progressive Vision Problem: Vision problem that gradually worsens over time.
Ptosis: Drooping of the upper eyelid (one or both) slightly or entirely over the pupil.
Pupil: The hole in the middle of the iris that appears black.
R Back to Top Reading Stand: Device that supports regular or large print books and allows the reader to change the position.
Reduced (or absent) Color Sensitivity: Vision problem that causes color confusion and night blindness.
Reduced Contrast Sensitivity: Vision problem that limits the eye's ability to distinguish between the lightness and darkness of objects.
Refraction: 1. Deviation or bending of rays of light when passing from one clear substance to another of a different density. 2. The determination of refractive errors in the eye and their correction with lenses.
Refractive Error: Condition that occurs when light rays entering the eye are not bent enough or are bent too much, so that they are not brought to a sharp focus on the retina. This results in myopia (nearsightedness), hyperopia (farsightedness), or astigmatism.
Residual Vision: Vision available to an individual who is not totally blind.
Retina: Thin sensory tissue which lines the back of the eye. It contains millions of photoreceptors (rods and cones) that capture light and convert them into electrical impulses. These impulses travel along the optic nerve to the brain where they are turned into images.
Retinal Detachment: Separation of the inner layer of the retina from the outer layer, resulting in loss of retinal function.
Retinitis CMV (Cytomegalovirus): Virus that leads to a variety of effects on the fetus. Vision problems include blurred vision, photophobia, distorted images, and central blind spots.
Retinitis Pigmentosa: A rare hereditary disease which is characterized by retinal degeneration and progressive blindness. It begins with night blindness in adolescence or in adult life.
Retinoblastoma: Rare, slow growing tumor that is located on the retina of the eye.
Retinopathy of Prematurity (ROP): Destructive retinal changes that sometimes develop after prolonged life-sustaining oxygen therapy is given to a premature infant. These changes can result in a minimal loss of vision to a total loss of vision in one or both eyes.
Rod: Part of the photoreceptors that are primarily responsible for peripheral and night vision and provide information about movement and shapes. There are approximately 125 million rods in the human eye.
Rubella: Infectious disease known as German measles. When pregnant women in the first trimester contract rubella, it affects the fetus in the areas of vision, hearing, and heart.
S Back to Top Saccadic Movements: Quick movements of both eyes together in the same direction for scanning objects and fixation.
Sclera: Tough outside layer of the eyeball, which is sometimes called the "white of the eye"; made up of crisscrossing fibers. Serves as the eye's protective outer coat.
Scotoma: Field loss that consists of isolated blind spots.
Septo Optic Dysplasia or De Morsier's Syndrome: Syndrome characterized by growth hormone deficiencies and visual deficits, which range from normal visual acuity to subtle visual field changes, to no light perception.
Strabismus: Eye disorder in which the eye turns inward or outward. Caused by one or more improperly functioning eye muscles.
Superior Oblique: One of four muscles that controls the vertical motion of the eyeball. Rotates the eye so that the top of the eye moves towards the nose.
Superior Rectus: One of four muscles that controls the vertical motion of the eyeball. Moves the eye up.
T Back to Top Tunnel Vision: Eye condition in which the field of vision is 20 degrees or less.
V Back to Top Vision: Ability of the brain to process information received from the eye.
Vision Stimulation: Series of activities and exercises designed to increase an individual's visual capabilities.
Visual Acuity: Ability of the eyes to distinguish object details and shape. Common visual activity measurement is 20/20.
Visual Cortex: Part of the brain that processes visual information.
Visual Discrimination: Ability to match or determine exact characteristics of two forms when one of the forms is among similar forms.
Visual Field: Space visible to the eyes when they are fixating straight ahead.
Visual Processing Problem: Vision problem often caused by brain damage. The eyes collect and transmit information correctly, but the brain does not process it precisely.
Visual Tracking: Ability to follow the movement of an object with the eyes.
Vitreous Humor: Clear, jelly-like substance that fills the posterior chamber and keeps the eye shaped like a ball.

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