Source:allaboutvision
Strabismus is a failure of the two eyes to maintain proper alignment and work together as a team.
If you have strabismus, one eye looks directly at the object you are viewing, while the other eye is misaligned inward (esotropia, "crossed eyes" or "cross-eyed"), outward (exotropia or "wall-eyed"), upward (hypertropia) or downward (hypotropia).
Strabismus can be constant or intermittent. The misalignment also might always affect the same eye (unilateral strabismus), or the two eyes may take turns being misaligned (alternating strabismus).
To prevent double vision from congenital and early childhood strabismus, the brain ignores the visual input from the misaligned eye, which typically leads to amblyopia or "lazy eye" in that eye.
According to the American Association for Pediatric Ophthalmology and Strabismus, approximately 4 percent of the U.S. population has crossed eyes or some other type of strabismus.