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