1) Involuntary contraction of the muscle fibers innervated by a motor unit. Fasciculations can often by visualized and take the form of a muscle twitch or dimpling under the skin, but usually do not generate sufficient force to move a limb. They may represent a benign condition or occur as a manifestation of motor neuron disease or peripheral nervous system diseases. (Adams et al., Principles of Neurology, 6th ed, p1294)

2) Muscular twitching involving the simultaneous contraction of contiguous groups of muscle fibers.