Wordnet 3.0
ADJECTIVE (1)
1. 
 of or relating to abnormal neuromuscular activity characterized by rapidly alternating muscle contraction and relaxation; 
- Example: "clonic spasm"
The Collaborative International Dictionary of English v.0.48:
Clonic \Clon"ic\, a. [Gr. klo`nos a violent, confused motion;
   cf. F. clonique.] (Med.)
   Having an irregular, convulsive motion. --Dunglison.
   [1913 Webster]
   Clonic spasm. (Med.) See under Spasm.
      [1913 Webster]
WordNet (r) 3.0 (2006):
clonic
    adj 1: of or relating to abnormal neuromuscular activity
           characterized by rapidly alternating muscle contraction
           and relaxation; "clonic spasm"