Source : WordNet®

pacemaker
     n 1: a leading instance in its field; "the new policy will be a
          pacesetter in community relations" [syn: {pacesetter}]
     2: a specialized bit of heart tissue that controls the
        heartbeat [syn: {cardiac pacemaker}, {sinoatrial node}, {SA
        node}]
     3: an implanted electronic device that takes over the function
        of the natural cardiac pacemaker [syn: {artificial
        pacemaker}]
     4: a horse used to set the pace in racing [syn: {pacer}, {pacesetter}]