ar·rhyth·mia noun \ā-ˈrith-mē-ə\: an alteration in rhythm of the heartbeat either in time or force.
(Dysrhythmia is actually a more precise term, as arrhythmia, strictly speaking, means absence of rhythm...)
New Latin, from Greek, lack of rhythm, from arrhythmos unrhythmical, from a- + rhythmos rhythm
First Known Use: circa 1860
There are many different kinds of arrhythmias, some benign and transient (palpitations caused by excitement, caffeine, or lack of sleep--these are usually PACs or PVCs), some serious, others lethal, such as V-tach or V-fib.
(normal sinus rhythm, shown here for comparison)
(a very distinct-looking arrhythmia, atrial flutter; note the sawtooth-shaped baseline)
(and a lethal arrhythmia, ventricular tachycardia)