One that sets the pace for another; one that takes the lead or sets an example

A medical device implanted under the skin of a patient whose heart beats inefficiently. It delivers minute electric shocks to stimulate the heart muscles at regular intervals and restores normal heartbeat.
The latest pacemakers are powered by radioactive isotopes for long life and weigh no more than 15 g/0.5 oz.
An implanted electronic device that takes over the function of the natural cardiac pacemaker; SYN. artificial pacemaker.

Or sinoatrial node (SA node) In vertebrates, a group of muscle cells in the wall of the heart that contracts spontaneously and rhythmically, setting the pace for the contractions of the rest of the heart. The pacemaker's intrinsic rate of contraction is increased or decreased, according to the needs of the body, by stimulation from the autonomic nervous system. A specialized bit of heart tissue that controls the heartbeat; SYN. cardiac pacemaker, sinoatrial node, SA node.