Pacemaker
Part of speech: noun
Definition: One who sets the pace in a race, to guide the others. A set of nerves which stimulate the heart to beat. A medical implement that is used to stimulate a heart to beat by simulating the action of the natural pacemaker.
Example sentence: I suspect the psychological pressure associated with that crisis caused the first mental blackout I had ever suffered. It contributed to a deterioration in my health that later required the insertion of a heart pacemaker.
Pacesetter
Part of speech: noun
Definition: A person who determines the rate of action through leading.