What term is used to describe an abnormal heart rhythm?

The term used to describe an abnormal heart rhythm is "arrhythmia." This term encompasses a variety of irregular heartbeats, which may manifest as a heart beating too fast, too slow, or in an uncoordinated manner.

While terms like bradycardia and tachycardia describe specific types of arrhythmias—bradycardia refers to a slower than normal heart rate, and tachycardia refers to a faster than normal heart rate—arrhythmia itself is the broader category that includes any deviation from the normal heart rhythm. Fibrillation, on the other hand, specifically refers to a rapid and chaotic heartbeat that can be life-threatening, but it too falls under the umbrella of arrhythmia.

Thus, referring to the general concept of an abnormal heart rhythm, "arrhythmia" is the most accurate term.