Which medical term describes a rapid heart rate?

The term that describes a rapid heart rate is "tachycardia." In medical terminology, "tachy-" refers to a fast or rapid rate, while "-cardia" relates to the heart. Therefore, tachycardia specifically indicates an increased heart rate, typically defined as a resting heart rate over 100 beats per minute in adults.

Bradycardia is the opposite condition, referring to a slower than normal heart rate. Arrhythmia describes any irregular heartbeat, which may be rapid, slow, or erratic, but does not specifically denote a rapid heart rate. Cardiomyopathy refers to diseases of the heart muscle, which can affect its size and function but does not specifically address heart rate. Thus, tachycardia is the appropriate term for a rapid heart rate.