Final answer:
Atrial fibrillation is a condition in which the atria beat at a fast and irregular rate.
Step-by-step explanation:
Atrial fibrillation is a condition characterized by the irregular and rapid beating of the atria, the upper chambers of the heart. In atrial fibrillation, the atria can beat at a rate between 350 and 700 impulses per minute. This is much faster than the normal resting heart rate of 60-100 beats per minute.