Original: Ventricular fibrillation (V-fib or VF) is a condition in which there is uncoordinated contraction of the cardiac muscle of the ventricles in the heart, making them tremble rather than contract properly.
Simplified: Ventricular fibrillation (often shortened to VF or V-Fib) is a cardiac arrhythmia in which there is an uncoordinated contraction of the cardiac muscle of the ventricles in the heart.