Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

nystagmus
     n : involuntary movements of the eyeballs; the presence or
         absence of nystagmus is used to diagnose a variety of
         neurological and visual disorders