Final answer:
Atrial fibrillation (A fib) is the condition characterized by an irregular and rapid heartbeat. It is a serious type of arrhythmia.
Step-by-step explanation:
The condition characterized by an irregular and rapid heartbeat, often referred to as A fib, is Atrial fibrillation is a serious condition in which the atria of the heart beat chaotically and irregularly. It is one of the types of arrhythmia, which is a group of conditions in which the heartbeat is irregular, too fast, or too slow.