What defines an arrhythmia?

An arrhythmia is characterized by an irregular heartbeat, which can manifest as fluctuations in the timing or rhythm of the heart's contractions. This condition indicates that the heart is not beating in a normal or predictable pattern, which can lead to various symptoms or complications, depending on the severity and type of arrhythmia.

A regular heart rate, high heart rate, or low heart rate all describe conditions related to the heart's rhythm or rate, but they do not capture the essence of what constitutes an arrhythmia, which is fundamentally about irregularity. Therefore, the definition of an arrhythmia aligns closely with the notion of an irregular heartbeat, explaining why this choice stands out as the correct answer.