pacemaker

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Sports One who sets the pace in a race. Also called pacer, pacesetter.
2. n. A leader in a field: the fashion house that is the pacemaker. Also called pacesetter.
3. n. 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.
4. n. Any of several usually miniaturized and surgically implanted electronic devices used to stimulate or regulate contractions of the heart muscle.
5. n. Biochemistry A substance that regulates a series of related reactions.

Century Dictionary and Cyclopedia

1. n. One who sets the pace for others, as in racing.
2. n. One who or that which makes a record for speed.

Wiktionary

1. n. One who sets the pace in a race, to guide the others.
2. n. anatomy A set of nerves which stimulate the heart to beat.
3. n. hence, medicine A medical implement that is used to stimulate a heart to beat by simulating the action of the natural pacemaker.

GNU Webster's 1913

1. n. (Physiol. & Anat.) a specialized bit of heart tissue that controls the heartbeat.
2. n. An implanted electronic device that takes over the function of the natural cardiac pacemaker{1}; -- used to assist people whose heartbeat is irregular.
3. n. A horse used to set the pace in racing.

WordNet 3.0

1. n. an implanted electronic device that takes over the function of the natural cardiac pacemaker
2. n. a specialized bit of heart tissue that controls the heartbeat
3. n. a horse used to set the pace in racing
4. n. a leading instance in its field