pacemaker

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Sports One who sets the pace in a race.
• noun A leader in a field.
• noun 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.
• noun Any of several usually miniaturized and surgically implanted electronic devices used to stimulate or regulate contractions of the heart muscle.
• noun Biochemistry A substance that regulates a series of related reactions.

from The Century Dictionary.

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

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

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

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