[fahy-bruh-ley-shuhn, or especially for 2, fib-ruh-]
the formation of fibrils.
Pathology. uncontrolled twitching or quivering of muscular fibrils.

1830–40; fibrillate + -ion Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
fibrillation (ˌfaɪbrɪˈleɪʃən, ˌfɪb-)
1.  a local and uncontrollable twitching of muscle fibres, esp of the heart, not affecting the entire muscle. Atrial fibrillation results in rapid and irregular heart and pulse rate. In ventricular fibrillation, the heart stops beating
2.  irregular twitchings of the muscular wall of the heart, often interfering with the normal rhythmic contractions

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

c.1840, from fibrillate + -tion.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

fibrillation fib·ril·la·tion (fĭb'rə-lā'shən, fī'brə-)

  1. Fine, rapid twitching of individual muscle fibers with little or no movement of the muscle as a whole.

  2. The formation of fibrils.

  3. Vermicular twitching, usually slow, of individual muscular fibers, commonly occurring in the atria or ventricles of the heart and in recently denervated skeletal muscle fibers.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
fibrillation   (fĭb'rə-lā'shən)  Pronunciation Key 
A rapid twitching of muscle fibers, as of the heart, that is caused by the abnormal discharge of electrical nerve impulses. Ventricular fibrillation is life-threatening.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
In atrial fibrillation, the atrium or upper chamber of the heart does not beat
  an organized manner.
Slayton had atrial fibrillation, which had never been debilitating, but the
  flight surgeons grounded him anyway.
The surgical tool has performed well in animal tests designed to mimic a
  disorder called atrial fibrillation.
All things being equal, atrial fibrillation is a relatively benign condition as
  long as it can be controlled.
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