Source : Webster's Revised Unabridged Dictionary (1913)

Clonus \Clo"nus\, n. [NL., fr. Gr. ? violent, confused motion.]
   (Med.)
   A series of muscular contractions due to sudden stretching of
   the muscle, -- a sign of certain neuropathies.

Source : WordNet®

clonus
     n : convulsion characterized by alternating contractions and
         relaxations