n 1  :  an act or process of forming fibers or fibrils  2 a  :  a muscular twitching involving individual muscle fibers acting without coordination  b  :  very rapid irregular contractions of the muscle fibers of the heart resulting in a lack of synchronism between heartbeat and pulse - see ATRIAL FIBRILLATION VENTRICULAR FIBRILLATION  .
 
   
 
Published under license with Merriam-Webster, Incorporated.  © 1997-2004.