pace·mak·er 
(pāsmā′kər)
(pāsmā′kər)n.
1. Sports One who sets the pace in a race. Also called pacer, pacesetter.
2. A leader in a field: the fashion house that is the pacemaker. Also called pacesetter.
3.
a. A part of the body, such as the mass of muscle fibers of the sinoatrial node, that sets the pace or rhythm of physiological activity.
b. Any of several usually miniaturized and surgically implanted electronic devices used to stimulate or regulate contractions of the heart muscle.
pacemak′ing adj. & n.