Irregular heartbeat. An arrhythmia is a problem with the rate or rhythm of the heart. The heart can beat too fast (tachycardia), too slow (bradycardia), or irregularly. Many arrhythmias are harmless, but some can be serious or even life threatening. There are many types, and your doctor can explain which particular problem you have, and the treatment options.
Irregular rhythm. An arrhythmia is when the heart does not beat in the normal regular pattern that it usually does. This can be benign (where you have an occasional extra beat every now and then), to severe (a life-threatening irregular rhythm where the heart can not pump enough blood).