pacemaker n. (Physiol. & Anat.) 1. a specialized bit of heart tissue that controls the heartbeat.Syn. -- cardiac pacemaker, sinoatrial node.[WordNet 1.5]
2. An implanted electronic device that takes over the function of the natural cardiac pacemaker{1}; -- used to assist people whose heartbeat is irregular.Syn. -- artificial pacemaker.[WordNet 1.5]
3. A horse used to set the pace in racing.Syn. -- pacer, pacesetter.[WordNet 1.5]