noun (n)

• a leading instance in its field(noun.cognition)

  Synonym:
  pacesetter
  

  source: wordnet30
• a specialized bit of heart tissue that controls the heartbeat(noun.body)

  Synonym:
  cardiac pacemaker, sa node, sinoatrial node
  

  source: wordnet30
• an implanted electronic device that takes over the function of the natural cardiac pacemaker(noun.artifact)

  Synonym:
  artificial pacemaker
  

  source: wordnet30
• a horse used to set the pace in racing(noun.animal)

  Synonym:
  pacer, pacesetter
  

  source: wordnet30