Atrial fibrillation: An abnormal and irregular heart rhythm in which electrical signals are generated chaotically throughout the upper chambers (atria) of the heart. Many people with atrial fibrillation have no symptoms. Among those who do, the most common symptom is an uncomfortable awareness of the rapid and irregular heartbeat (palpitations). Atrial fibrillation can promote the formation of blood clots that travel from the heart to the brain, resulting in stroke. Treatment of atrial fibrillation involves risk-factor control, use of medications to slow the heart rate and/or convert the heart to. normal rhythm, and prevention of blood clots. Also known as auricular fibrillation.