An irregular heartbeat or abnormal rhythm of the heart; a medical condition where the heart beats too fast, too slow, or erratically.
From Greek 'a-' (without) and 'rhythmos' (rhythm), literally meaning 'without rhythm.' The medical term was formalized in the 19th century as cardiology developed.
Your heart naturally varies its rhythm throughout the day—speeding up when you're excited, slowing down when you sleep—so doctors had to define which irregularities are actually dangerous arhythmias versus normal variation!
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