Irregular, spontaneous, involuntary, and usually continual and uncoordinated contractions of a single muscle group that is supposed to be at rest: Unlike fibrillation, which is a similar condition, fasciculation can be seen under the skin and it is described as either minimal or major in movements.

Fasciculations can occur in normal individuals without an associated disease or condition, or as a result of illness; such as, muscle cramps, nerve diseases, and metabolic imbalances.

Persistent fasciculations with weakness in the affected muscle can be diagnosed as damage to, or a disease, of the nerve cells in the spine which control the muscle or nerve fibers that connect the spinal nerves to the muscle.

One of the ailments of the disorder of fasciculations are "motor neuron diseases" in which groups of nerves that control muscular activities degenerate within the brain and spinal cord; resulting in weakness and atrophy (wasting away) of the muscles.

The fasciculation of the heart muscle is known as "fibrillation".