pacemaker

from The American Heritage® Dictionary of the English Language, 4th Edition

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

from Wiktionary, Creative Commons Attribution/Share-Alike License

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

from the GNU version of the Collaborative International Dictionary of English

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

from The Century Dictionary and Cyclopedia

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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