Fibrillation refers to an irregular and rapid contraction of the muscles or the heart. This abnormal pattern of movement can disrupt the normal functioning of the affected muscle. For example, atrial fibrillation is a type of fibrillation that occurs in the atria (upper chambers) of the heart and can lead to an erratic [irregular] heartbeat.
Very Simple Definition:
Fibrillation is when the muscles or heart have a fast and irregular movement. For example, sometimes the heart can beat really fast and not in a regular pattern.