The Collaborative International Dictionary of English v.0.48:

Arbitrator \Ar"bi*tra`tor\, n. [L., fr. arbitrari: cf. F.
   arbitrateur.]
   1. A person, or one of two or more persons, chosen by parties
      who have a controversy, to determine their differences.
      See Arbitration.
      [1913 Webster]

   2. One who has the power of deciding or prescribing without
      control; a ruler; a governor.
      [1913 Webster]

            Though Heaven be shut,
            And Heaven's high Arbitrators sit secure. --Milton.
      [1913 Webster]

            Masters of their own terms and arbitrators of a
            peace.                                --Addison.
      [1913 Webster]

   Syn: Judge; umpire; referee; arbiter. See Judge.
        [1913 Webster]

WordNet (r) 3.0 (2006):

arbitrator
    n 1: someone chosen to judge and decide a disputed issue; "the
         critic was considered to be an arbiter of modern
         literature"; "the arbitrator's authority derived from the
         consent of the disputants"; "an umpire was appointed to
         settle the tax case" [syn: arbiter, arbitrator,
         umpire]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "arbitrator":
   JP, Justice, arbiter, bencher, broker, connection, contact, critic,
   go-between, his honor, his lordship, his worship,
   impartial arbitrator, indicator, interagent, intermediary,
   intermediate, intermedium, internuncio, interpleader, judge,
   justice, magistrate, mediator, medium, middleman, moderator,
   negotiant, negotiator, referee, third party, umpire,
   unbiased observer