NOUN (1)

1. involuntary movements of the eyeballs; its presence or absence is used to diagnose a variety of neurological and visual disorders;

The Collaborative International Dictionary of English v.0.48:

Nystagmus \Nys*tag"mus\, n. [NL., fr. Gr. ? drowsiness, fr. ? to
   nod in sleep, to slumber.] (Med.)
   A rapid involuntary oscillation of the eyeballs.
   [1913 Webster]