Nystagmus is an involuntary rapid tracking movement of the eyes. It usually affects both eyes - and is often exaggerated or stopped by looking in a particular direction. The eyes may move from side to side (horizontal nystagmus), up and down (vertical nystagmus), circular (rotatory nystagmus) or a combination of these.

The most common types are motor and sensory nystagmus. Motor nystagmus is usually hereditary and present shortly after birth. Sensory nystagmus is an acquired condition and if the underlying cause is known it may be treatable. It usually begins in infancy.