Extrasystole: A premature contraction of the heart that is independent of the normal rhythm of the heart and that arises in response to an impulse in some part of the heart other than the normal impulse from the sinoatrial (SA) node. The extrasystole is followed by a pause, as the heart electrical system "resets" itself and the contraction following the pause is usually more forceful than normal. These more forceful contractions are frequently perceived as palpitations.
Systole refers to the time when the left ventricle of the heart contracts. Extrasystole is also known as extra systole, premature beat, premature contraction, premature systole, premature ventricular beat or PVB, premature ventricular contraction or PVC.