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 ]