Terminology, defined once.

A

AC/A Ratio — The ratio of accommodative convergence to accommodation; describes how much convergence is driven reflexively by a given amount of focusing effort. A key factor in whether plus or minus lenses help or worsen a given phoria. See Accommodative Dysfunction.

Accommodation — The ability to change focus between different distances, achieved by the brain sending signals through the autonomic nervous system to the ciliary muscle inside the eye, to adjust the shape of the lens. See Accommodative Dysfunction.

Accommodative Facility — How efficiently and quickly a patient can shift focus between near and far, typically measured with plus/minus lens flippers. See Accommodative Dysfunction.

Accommodative Excess — Difficulty relaxing accommodation appropriately, often producing intermittent distance blur after sustained near work. One of the three accommodative dysfunction subtypes. See Accommodative Dysfunction.

Accommodative Infacility — Difficulty shifting focus efficiently between near and far, regardless of accommodative amplitude. One of the three accommodative dysfunction subtypes. See Accommodative Dysfunction.

Accommodative Insufficiency — Reduced accommodative amplitude relative to age-expected norms. One of the three accommodative dysfunction subtypes. See Accommodative Dysfunction.

Amblyopia — A disorder of binocular development most characterized by reduced maximal visual acuity, intense cortical suppression, and other abnormalities of visual function. Results from early-onset strabismus, an uncorrected refractive error (whether of a large value in both eyes or a significant difference between the two eyes), or anything blocking clear image formation early in development. See Strabismus & Amblyopia.

Anisometropia — A significant refractive difference between the two eyes, a common cause of refractive amblyopia. See Strabismus & Amblyopia.

B

Base-In / Base-Out — Prism orientation. Base-in prism shifts an image outward (relieving convergence demand); base-out shifts it inward (relieving divergence demand, or stressing convergence during testing). See Convergence Insufficiency.

Binocular Vision Dysfunction (BVD) — The umbrella term for when the two eyes fail to work together effectively, whether through an uncomfortable phoria or difficulty dynamically integrating the two eyes despite a straight resting posture. Strabismus and amblyopia are themselves categories within this broader umbrella. See Binocular Vision Dysfunction.

C

CA/C Ratio — The ratio of convergence accommodation to convergence; the reverse relationship of the AC/A ratio, describing how much accommodation is driven reflexively by a given amount of convergence effort. See Binocular Vision Dysfunction.

CISS (Convergence Insufficiency Symptom Survey) — A standardized symptom questionnaire used to quantify near-work-related symptoms as part of a CI diagnosis. See Convergence Insufficiency.

CITT (Convergence Insufficiency Treatment Trial) — A large NIH-funded randomized trial establishing office-based vision therapy with home reinforcement as the most effective treatment for CI. See Convergence Insufficiency.

Convergence — The inward turning of both eyes to maintain single vision on a near target.

Convergence Insufficiency (CI) — A binocular vision disorder in which the eyes struggle to turn inward and sustain alignment on a near target. See Convergence Insufficiency.

Cover Test — A diagnostic technique in which one eye is covered and observed for movement upon uncovering (or the fellow eye is observed for movement), used to detect and measure phorias and tropias. The alternate cover test dissociates both eyes to reveal the total deviation; the unilateral cover test detects manifest strabismus specifically.

Cyclophoria — A latent rotational (torsional) heterophoria, often producing a perceived tilting of the visual world. See Binocular Vision Dysfunction.

Cyclotropia — A manifest rotational eye misalignment, the tropia counterpart to cyclophoria. See Binocular Vision Dysfunction.

D

Decompensation — When a phoria breaks down into a manifest deviation, producing diplopia. Technically a category of strabismus, most relevant in older patients or very high phorias. See Binocular Vision Dysfunction.

DEM Test (Developmental Eye Movement Test) — A standardized test of saccadic eye movement accuracy and speed, commonly used in evaluating oculomotor dysfunction. See Oculomotor Dysfunction & Reading.

Deprivation Amblyopia — Reduced acuity from anything physically blocking clear image formation early in development — congenital cataract, ptosis, corneal opacity. The most urgent amblyopia subtype, requiring prompt intervention in infancy. See Strabismus & Amblyopia.

Diplopia — Double vision; the perception of two images of a single object, occurring when the two eyes' images fail to fuse into one.

Divergence — The outward turning of both eyes, most relevant for distance fixation or relieving excess convergence.

E

Esophoria — A latent inward-turning tendency of the eyes.

Esotropia — A manifest inward eye turn, the tropia counterpart to esophoria. See Strabismus & Amblyopia.

Exophoria — A latent outward-turning tendency of the eyes.

Exotropia — A manifest outward eye turn, the tropia counterpart to exophoria. See Strabismus & Amblyopia.

F

Fixation — The ability to hold gaze steady on a stationary point long enough for the brain to extract visual information. See Oculomotor Dysfunction & Reading.

Fixation Disparity — A small, typically imperceptible misalignment that persists even under binocular viewing conditions; fluctuating fixation disparity under sustained demand can indicate fusion control fatigue. See Binocular Vision Dysfunction.

Fusional Reserve — The measured range of vergence available to a patient in a given direction (base-in or base-out), indicating how much visual stress the system can absorb before fusion breaks. See Binocular Vision Dysfunction.

Fusional Vergence — The eyes' capacity to converge or diverge in order to maintain single, fused vision despite a phoria.

H

Heterophoria — A latent deviation of the eyes, present but not manifest under normal binocular viewing. Says nothing on its own about symptoms or clinical significance. See Binocular Vision Dysfunction.

Hyperphoria / Hypophoria — A latent upward (hyper) or downward (hypo) deviation of one eye relative to the other. See Binocular Vision Dysfunction.

Hypertropia / Hypotropia — A manifest upward (hyper) or downward (hypo) deviation of one eye relative to the other, the tropia counterpart to hyperphoria/hypophoria. See Binocular Vision Dysfunction.

M

Maddox Rod — A testing device consisting of parallel cylindrical lenses that turns a point light source into a line, used to measure the direction and magnitude of a phoria without a fusable target.

MEM Retinoscopy — Monocular Estimate Method retinoscopy; a technique for objectively assessing a patient's accommodative response and lag in real time. See Accommodative Dysfunction.

N

Near Point of Convergence (NPC) — The closest point at which the eyes can maintain single, fused vision on an approaching target; a receded NPC is a core finding in convergence insufficiency. See Convergence Insufficiency.

NRA/PRA (Negative/Positive Relative Accommodation) — A measurement of the range of clear vision available at near through plus (NRA) and minus (PRA) lenses while convergence is held fixed. Useful but incomplete on its own, since it doesn't reveal accommodative posture, lag, or facility. See Accommodative Dysfunction.

NSUCO — A standardized oculomotor rating scale (Northeastern State University College of Optometry) assessing saccades and pursuits as separate, independently scored skills. See Oculomotor Dysfunction & Reading.

P

Panum's Fusional Area — The range of retinal disparity within which the brain can fuse two images without diplopia; substantially narrower vertically than horizontally, which is why small vertical deviations often produce disproportionate symptoms. See Binocular Vision Dysfunction.

Percival's Criterion — A formula historically used to guide prism prescribing; widely regarded in this practice as unreliable and prone to overprescribing. See Binocular Vision Dysfunction.

Phoria — See Heterophoria. Best understood not as a deviation to be corrected, but as a readout of the resting posture the brain has adopted to minimize effort. See Binocular Vision Dysfunction.

Prism Diopter (PD or Δ) — The unit of measurement for the optical power of prism, and for the magnitude of a phoria or tropia.

Pursuits — The smooth tracking eye movement used to follow a moving target; the only eye movement system that cannot be voluntarily generated without an actual moving stimulus. See Oculomotor Dysfunction & Reading.

R

Refractive Amblyopia — Reduced acuity from an uncorrected significant refractive difference between the eyes, without visible misalignment. Often detected later than strabismic amblyopia since there's no visible sign prompting an exam. See Strabismus & Amblyopia.

S

Saccades — Rapid, voluntary jumps between fixation points, most relevant to reading as the eyes move across a line of text. See Oculomotor Dysfunction & Reading.

Sheard's Criterion — A formula historically used to guide prism prescribing based on the relationship between phoria and fusional reserve; widely regarded in this practice as unreliable and prone to overprescribing. See Binocular Vision Dysfunction.

Stereoacuity — The finest degree of depth (stereopsis) a patient can discriminate, typically measured in arcseconds; used as a quality barometer for how well the two eyes are working together.

Stereopsis — Depth perception derived from the brain fusing the two eyes' slightly different retinal images. See Binocular Vision Dysfunction.

Strabismic Amblyopia — Reduced acuity in the deviating eye from chronic suppression during strabismus, with measurably reduced cortical territory for that eye. See Strabismus & Amblyopia.

Strabismus — A manifest eye misalignment, visibly present rather than latent. See Strabismus & Amblyopia.

Suppression — An active, adaptive cortical process in which the brain reduces or ignores input from one eye to avoid double vision or visual confusion; central to both amblyopia and strabismus. See Strabismus & Amblyopia.

T

Tropia — A manifest, constant or intermittent eye misalignment. See Strabismus.

V

Vergence — The general term for coordinated eye movement in opposite directions to align both eyes on a target at a given depth; convergence and divergence are its two components.

Vergence Facility — How efficiently and repeatedly the eyes can shift convergence demand without losing fusion, typically tested with alternating prism flippers. A dynamic, stamina-based measure distinct from static vergence ranges. See Convergence Insufficiency.

Vestibular System — The inner-ear sensory system that detects head movement and position, working continuously with vision and proprioception to maintain balance and stable gaze. See Visually Induced Dizziness.

Vestibulo-Ocular Reflex (VOR) — The reflex that stabilizes gaze during head movement by driving compensatory eye movement in the opposite direction. See Visually Induced Dizziness.

Vision Therapy (VT) — Structured, office-based and home-reinforced treatment that directly trains visual skills — binocular coordination, accommodation, oculomotor control — rather than optically compensating for a deficit. See Vision Therapy.

Visual-Vestibular Integration — The continuous coordination between the visual and vestibular systems that supports stable gaze and balance; disruption in either system can produce dizziness or motion sensitivity even when the other tests completely normal. See Visually Induced Dizziness.

VO Star — A clinical testing target used to assess sustained vergence ranges and binocular stability under dynamic conditions. See Binocular Vision Dysfunction.

W

Worth 4 Dot Test — A test using colored, filtered targets to assess whether a patient is fusing, suppressing, or experiencing diplopia at distance and near. See Binocular Vision Dysfunction.