Involuntary contraction (5)
(Other definitions for spasm that I've seen before include "Involuntary muscular contraction" , "Sudden convulsion" , "Burst" , "cramp" , "Fit" .)
(Other definitions for cramp that I've seen before include "Confine - muscular spasm" , "Form of muscular pain" , "Muscle spasm" , "common among writers" , "writers may be familiar with" .)