Strabismus: A condition in which the visual axes of the eyes are not parallel and the eyes appear to be looking in different directions. In divergent strabismus, or exotropia, the visual axes diverge. If the visual axes converge, it is called convergent strabismus or esotropia. The danger with strabismus is that the brain cones may come to rely more on one eye than the other and that part of the brain circuitry connected to the less-favored eye fails to develop properly, leading to amblyopia (blindness) in that eye.

The classic treatment for mild-to-moderate strabismus has long been an eyepatch, covering the stronger eye with a patch, forcing the weaker eye to do enough work to catch up. However, eyedrops can work as well as an eyepatch in correcting moderate lazy eye and preventing the development of amblyopia (blindness). Atropine eyedrops are instilled daily in the stronger (dominant) eye. The atropine works by blurring rather than blocking vision in the stronger eye.

Severe strabismus may require surgery. The surgery is designed to increase or decrease the tension of the small muscles outside the eye. (These muscles are called the extraocular eye muscles. The six extraocular eye muscles move the eye in all directions.) When strabismus surgery is needed, the sooner it is done, the better the chance of the child achieving normal binocular vision.

Adults sometimes also need strabismus surgery. This can be done in a standard manner, as in children. Or adjustable suture surgery may be done because scarring from old eye surgery, inflammation from eye muscle disease, or neurological eye weakness makes it difficult to gauge how much tension to take up or let out to straighten the eyes. With adjustable suture surgery, it is possible to adjust the tension of the muscles after the surgery.

Strabismus is also called cast, heterotropia, manifest deviation and squint. The term "strabismus" comes from the Greek "strabismos" = a squinting.