Involuntary shock-like contractions, irregular in rhythm and amplitude, followed by relaxation, of a muscle or a group of muscles. This condition may be a feature of some central nervous system diseases; (e.g., epilepsy, MYOCLONIC). Nocturnal myoclonus is the principal feature of the nocturnal myoclonus syndrome. (From Adams et al., Principles of neurology, 6th ed, pp102-3).