The Collaborative International Dictionary of English v.0.48:

Clonus \Clo"nus\, n. [NL., fr. Gr. klo`nos violent, confused
   motion.] (Med.)
   A series of muscular contractions due to sudden stretching of
   the muscle, -- a sign of certain neuropathies.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

clonus
    n 1: convulsion characterized by alternating contractions and
         relaxations