1. An abnormal rate of muscle contractions in the heart.
2. An irregularity in the rhythm of the heartbeat.

In an arrhythmia the heartbeats may be too slow, too rapid, too irregular, or too early.

Rapid arrhythmias (greater than 100 beats per minute) are called tachycardias. Slow arrhythmias (slower than 60 beats per minute) are called bradycardias. Irregular heart rhythms are called fibrillations (as in atrial fibrillation and ventricular fibrillation).

When a single heartbeat occurs earlier than normal, it is called a premature contraction.

3. Any variation from the normal rhythm of the heartbeat; it may be an abnormality of either the rate, regularity, or site of impulse origin or the sequence of activation.

The term encompasses abnormal regular and irregular rhythms as well as loss of rhythm.

Abnormal rates of muscle contractions or heart movements: With arrhythmias, a person's heartbeats may be too slow, too rapid, or very irregular.

Rapid arrhythmias (greater than 100 beats per minute) are called "tachycardias" while slow arrhythmias (slower than 60 beats per minute) are called "bradycardias"; and sometimes, irregular heart rhythms are called fibrillations or muscular twitching in the heart that involves individual muscle fibers that move without coordination.