Nystagmus is an involuntary jerking of the eye. Everyone has it to some extent, but it is usually abnormal when it occurs spontaneously (rather than being induced by gaze or positioning) or when looking at a stationary object.

The degree of nystagmus varies greatly between people and even in the same person at different times. Some things (such as alcohol and other central nervous system depressants) accentuate it and make it more visible. In some people, it is clearly visible naturally. There are dozens of different kinds of nystagmus. One can easily see nystagmus in someone else by having that person close their eyes and spinning them around. After a few spins, have the person stop and open their eyes. A distinct jerking of the eyes -- from side to side -- will be seen: this is rotational nystagmus. Strobe lights also produce nystagmus. It is also a symptom of many diseases.