Strabismus (also called squint and crossed eye), is a condition in which the eyes do not properly align with each other when looking at an object. Strabismus can be constant (always present) or intermittent (also called transient strabismus).
Strabismus is technically a broad term for a misaligned eye. Tropia is more specific. In medical terminology, the direction of the deviation is also provided: exotropia (eye turns out), esotropia (eye turns in), hypertropia (eye turns up), hypotropia (eye turns down).
A phoria is a latent (hidden) eye deviation. The eyes appear to be straight, but when covering an eye and breaking fusion, the eyes assume a position away from normal alignment. Most people have a very small phoria if tested, but a large phoria makes it hard to keep the eyes aligned, especially after a long day or when stressed. Phorias are also named with the exophoria, esophoria, hyperphoria, hypophoria convention.
Amblyopia (the medical term for lazy eye), is a condition in which the eye itself is health but vision is decreased. This can occur due to unequal glasses prescription (refractive error), eye turn (strabismus), or deprivation (lack of a visual stimulus when the eye is maturing).
Vergence is a broad term that relates to eye movement. For example, we can CONverg and DIverge the eyes.
A specific type of vergence disorder. The eyes are unable to maintain convergence for an extended period of time (reading a book for example). These patients often have a high exophoria, and may intermittantly lose fusion and show an exotropia, which causes double vision.
A prism diopter, often abbreviated with a greek delta (Δ) or ^ and represents a measure of angle, like degrees. Unlike degrees, it is not a linear unit, so it cannot be converted by simply multiplying a constant value.
The power of an ophthalmic prism (glass or plastic) in prism diopters (Δ) is equal to the deviation, in centimeters (cm), of a light ray passing through the prism, measured one meter, or 100 cm, away from the prism