pacemaker

(noun) a leading instance in its field. Synonyms: pacesetter. Example: "The new policy will be a pacesetter in community relations."

(noun) a specialized bit of heart tissue that controls the heartbeat. Synonyms: cardiac pacemaker, sinoatrial node, SA node.

(noun) an implanted electronic device that takes over the function of the natural cardiac pacemaker. Synonyms: artificial pacemaker.

(noun) a horse used to set the pace in racing. Synonyms: pacer, pacesetter.