Medical Definition of atrial fibrillation
: very rapid uncoordinated contractions of the atria of the heart resulting in a lack of synchronism between heartbeat and pulse beat <If you're experiencing chest pain, palpitations, lightheadedness or shortness of breath, you could have atrial fibrillation (AF), a condition in which the heart's upper chambers (atria) “quiver.”—Abigail L. Cuffey, Woman's Day, 10 Feb. 2010>—abbreviation AF; called also A-fib, auricular fibrillation
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