The Collaborative International Dictionary of English v.0.48:

pacemaker \pacemaker\ n. (Physiol. & Anat.)
   1. a specialized bit of heart tissue that controls the
      heartbeat.

   Syn: cardiac pacemaker, sinoatrial node.
        [WordNet 1.5]

   2. An implanted electronic device that takes over the
      function of the natural cardiac pacemaker[1]; -- used to
      assist people whose heartbeat is irregular.

   Syn: artificial pacemaker.
        [WordNet 1.5]

   3. A horse used to set the pace in racing.

   Syn: pacer, pacesetter.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

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