What word is best described by "irregular beat of the heart or other muscle"?

Fibrillation is an irregular, rapid, and uncoordinated contraction of muscle fibers, including the heart muscle. In the heart, disorganized electrical activity prevents the muscle from beating in a normal, coordinated rhythm.

The term is used most commonly in cardiology. Atrial fibrillation affects the heart’s upper chambers, or atria, producing an irregular rhythm that can reduce efficient blood flow and increase the risk of blood clots and stroke. Ventricular fibrillation affects the lower chambers and is a medical emergency because the heart can no longer pump blood effectively.

Fibrillation does not simply mean any unusually fast heartbeat. It describes chaotic or unsynchronized muscle activity. This is why the word can also refer to rapid, irregular contractions in muscles outside the heart, although cardiac conditions are the best-known examples.

A related term is flutter. Atrial flutter is generally more organized and regular than atrial fibrillation, even though both involve abnormal electrical activity. The stored answer matches the broad definition given in the question.