pacemaker pace·mak·er (pās'mā'kər)
A part of the body, such as the specialized mass of cardiac muscle fibers of the sinoatrial node, that sets the pace or rhythm of physiological activity.
Any of several usually miniaturized and surgically implanted electronic devices used to stimulate or regulate contractions of the heart muscle.
A substance whose rate or reaction sets the pace for a series of chain reactions.
The rate-limiting reaction itself.
A group of specialized muscle fibers in the heart that send out impulses to regulate the heartbeat. If the heart's built-in pacemaker does not function properly, an artificial pacemaker may be necessary — a small electrical device that also regulates the heartbeat by sending out impulses. An artificial pacemaker may be placed inside the body surgically or may be worn outside.