Arbitrator

Ar·bi·tra·tor

Arbitrator

n.
  1. A person, or one of two or more persons, chosen by parties who have a controversy, to determine their differences. See Arbitration.
  2. One who has the power of deciding or prescribing without control; a ruler; a governor.
    Though Heaven be shut, And Heaven's high Arbitrators sit secure.
    Masters of their own terms and arbitrators of a peace.