Source : Webster's Revised Unabridged Dictionary (1913)

Clonic \Clon"ic\, a. [Gr. klo`nos a violent, confused motion;
   cf. F. clonique.] (Med.)
   Having an irregular, convulsive motion. --Dunglison.

   {Clonic spasm}. (Med.) See under {Spasm}.

Source : WordNet®

clonic
     adj : of or relating to abnormal neuromuscular activity
           characterized by rapidly alternating muscle contraction
           and relaxation; "clonic spasm"