Strabismus
Strabismus, also known as crossed eyes, is a condition whereby the eyes fail to align correctly. One of the eyes can either turn inwards, outwards, upwards, or downwards as the other concentrates on a single point. It can occur all of the time or only on occasion.
Strabismus usually happens when the extraocular muscles, which regulate the eye and eyelid movement, aren't functioning together. Due to this, neither of the eyes can gaze at the same thing at the same time. This could also happen if the eyes are unable to coordinate properly as a result of a brain problem.
Because strabismus prevents binocular vision, it is difficult for the person to perceive depth perception.