Dictionary of terms relating to
eyesight and vision

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Depth Perception

The ability to judge relative distances of objects. (See "Stereopsis")

Depth Perception Test

A test to measure the ability of the vision system to discern the relative distances of various objects. (Also called a "Stereopsis Test")

Developmental Disorder

When a delay in an individual's normal development has occurred.

Developmental Vision Analysis

More comprehensive than a routine eye exam, examination will evaluate all of the patient's visual abilities such as visual acuity, eye focusing skills, eye teaming skills, eye tracking skills, visual motor skills, and visual perceptual skills.

Diopter (D)

A measurement of the refractive (light bending) power of a lens or a prism (pd). The strength of prescription glasses and contacts are measured in these units. For example a lens that is 0.50 diopter (D) is very weak, where as a lens that is 10.0 diopter (D) is very strong.


A single object is perceived as two rather than one; double vision.

Binocular Diplopia: Double vision can occur when the two eyes are not correctly aligned while aiming at an object, such as in esotropia and exotropia - these result in uncrossed diplopia and crossed diplopia, respectively (i.e. "crossed eyes", in esotropia, result in uncrossed diplopia). When the eyes are misaligned and aimed at different targets, two non-matching images are sent to the viewer's brain. When the viewer's brain accepts and use two non-matching images simultaneously, double vision results. Double vision is dangerous to survival, therefore, the brain naturally guards against its occurrence. In an attempt to avoid double vision, the brain can ignore one eye (suppression). Due to the brain's ability to suppress one eye, double vision can appear to go away without medical evaluation or treatment. The causes of the double vision are very likely still present and loss of vision in one eye can occur due to lack of treatment. The loss of vision in one eye can be temporary or permanent depending on detection and treatment. It is in this way, that diplopia contributes to loss of depth perception and binocular vision, amblyopia (lazy eye), and/or strabismus (deviating eye).

Monocular Diplopia: Diplopia can also occur when viewing with only one eye; this is called monocular diplopia, or where the patient perceives more than two images, monocular polyopia. In this case, the multiple vision can be caused by a structural defect in the vision system, such as cataracts, subluxation of the crystalline lens or Keratoconus causing irregularities in the refraction of light within the eye.

Temporary Diplopia:Temporary diplopia can also be caused by intoxication from alcohol or head injuries, such as concussion. If temporary double vision does not resolve quickly, one should see an eye doctor immediately. See also: polyopia

Direct Occlusion

Covering the non-amblyopic eye. (See "Inverse Occlusion" and "Occlusion")


Directionality relates to the awareness of the relationship of one object in space to another / laterality relates to the internal awareness of the two sides of the body. Directionality/Laterality can also be called spatial relations.

Directionality/Laterality Disorder

A condition in which an individual has poor development of left/right awareness. Symptoms of this disorder include confusion of right and left direction and letters and/or numbers reversals. Vision therapy is a helpful treatment option.

Distance Acuity

The eye's ability to distinguish an object's shape and details at a far distance such as 20 feet (6 meters).


The ability to use both eyes as a team and be able to turn the eyes out toward a far object.

Divergence Excess (DE) (clinical condition)

The eye's tendency to drift out relative to the direction of a distant object being viewed. Symptoms include: double vision at distance, headaches, eyestrain, nausea, dizziness, and blurred vision. Clinical signs include: exophoria greater at distance than near, high AC/A ratio, and reduced positive fusional vergence at distance. Can be improved with vision therapy.

Divergence Insufficiency (DI) (clinical condition)

The eye's tendency to turn more inward than necessary when viewing a distant object. Symptoms include: double vision, headaches, eyestrain, nausea, dizziness, and blurred vision. Clinical signs: esophoria greater at distance than near, low AC/A ratio, and reduced negative fusional vergence at distance. Treated with corrective lenses and vision therapy.

Dominant Eye

The eye that "leads" it partner during eye movements. Humans also have dominant hand, foot, eye, and side of the brain (not necessarily all on the same side).

Duction Test

A test of the eye's ability to turn inward or outward while maintaining single, binocular vision with the gradual introduction of progressively stronger base-in or base-out prisms.


Inability to "sound out" words and poor sight recognition of words. Dysphoneidesia is a subtype of dyslexia. Its characteristics are a combination of the other two forms of dyslexia: Dysphonesia and Dyseidetic.


Inability to "sound out" words. Dysphonesia is a subtype of dyslexia. Children with this form of dyslexia have difficulty sequentially analyzing and remembering what and where the sounds are in words. The resulting phonemic processing problems make it difficult to sound out new words, learn phonics, and make them dependent on their sight vocabulary. When they come to an unknown word they will often substitute a word using context clues. For example, "pony" for "horse", even though the substituted word doesn't look or sound anything like the original word. When spelling unknown words it is often difficult to determine what the original word is. For example, they may write "fmlue" for "familiar" or "lap" for "lamp". They cannot learn phonics because they cannot process where the sounds are. Their short term sequential auditory memory can be poor and result in repeating "8167" as "8671", or remember to go to their room but forgetting to get the item requested.


Poor sight recognition of words. Dyseidetic is a subtype of dyslexia. Children with this form of dyslexia have trouble analyzing and remembering written symbols. They continue to confuse the orientation. For example, they will write numbers and letters backwards long after other children have mastered these skills. They often confuse letter sequences in reading, and in spelling often get all the letters but in the wrong sequence (spelling "dose" for "does", "on " for "no", etc.). Their visual memory for words is poor, and after learning a new word they may fail to recognize that same new word later in the sentence. They have trouble learning to read and spell phonetically irregular words. For example, they may read " laugh" as "log" and spell it as "laff", both of which are phonetically consistent. Their spelling will have many mistakes, but will be phonetically consistent and one can usually tell what the word was they were trying to spell. When they are attempting to read an unknown word they will usually attempt to sound it out and do so very slowly.


A specific language-based disorder. The individual has difficulty with letter or word recognition, spelling, reading, writing, and sometimes naming pictures of objects. Dyslexia varies in degree from mild to very sever. It is caused by an inability of the brain's language centers to decode print or phonetically make the connection between the word's written symbols and their appropriate sounds. Dyslexia is not caused by a vision disorder. Children often are of normal or above normal intelligence. Dyslexia cannot be cured and will never be outgrown. Appropriate teaching methods can be taught to help those with dyslexia overcome their weakness. Click here for more information. The Dyslexia Determination test which is used by many optometrists who specialize in vision related vision problems investigates if the patient has one of the three forms of dyslexia: Dyseidetic - poor sight recognition of words, Dysphonesia- inability to "sound out" words, and Dysphoneidesia - a combination of characteristics from both types. Vision therapy is NOT considered a direct treatment for dyslexia.

Dystrophy (corneal)

Corneal dystrophy is a condition that causes a layer of the cornea to cloud over and impair visual clarity. It is usually a bilateral problem, which means it occurs in both eyes equally. There are more than 20 different forms of inherited corneal dystrophies. A corneal dystrophy can occur in otherwise healthy individuals. Depending on the type of condition and the age of the individual, a corneal dystrophy may either cause no problems, moderate vision impairment, or severe difficulties that require surgery. Gradual deterioration of the corneal tissue layers results in corneal dystrophy. As the tissue deteriorates, a gritty appearance such as that shown on the right, becomes apparent.