Nystagmus is a type of involuntary movement of the eyes. It is usually side-to-side (horizontal nystagmus) , but sometimes is up and down (vertical nystagmus) or in a circular fashion (rotatory nystagmus). The movement varies between slow and fast and usually involves both eyes. In infancy, it tends to develop between six weeks and three months of age and is called infantile nystagmus. It can also be acquired later in life and is called acquired nystagmus.

The direct cause of nystagmus is instability in the motor system that controls the eyes. In some cases, the cause of nystagmus is unknown.