noun

1. 1. a runner, horse, automobile, etc. that sets the pace for others, as in a race
   2. a person, group, or thing that leads the way or serves as a model
2. Anat.
   1. a dense network of interwoven, specialized muscle fibers in the right atrium of the heart that controls the rhythm of the electrical impulses that cause the heartbeat
   2. any of several body parts that control the rhythm of a biological activity, as an area in the stomach that controls stomach contractions
3. Med. an electronic device implanted into the body and connected to the wall of the heart, designed to provide regular, mild, electric shock that stimulates contraction of the heart muscles and restores normalcy to the heartbeat