Strabismus is a condition that interferes with binocular vision because it prevents a person from directing both eyes simultaneously towards the same fixation point; the eyes do not properly align with each other. Heterotropia is a medical synonym for the condition. Colloquial terms for strabismus include "cross-eye", "wall-eye", and a "cast of the eye". In British English it is sometimes referred to as a "squint", though that word more broadly is a verb meaning 'to narrow the eye opening', not related to strabismus.