The Collaborative International Dictionary of English v.0.48:

Arbiter \Ar"bi*ter\, n. [L. arbiter; ar- (for ad) + the root of
   betere to go; hence properly, one who comes up to look on.]
   1. A person appointed, or chosen, by parties to determine a
      controversy between them.
      [1913 Webster]

   Note: In modern usage, arbitrator is the technical word.
         [1913 Webster]

   2. Any person who has the power of judging and determining,
      or ordaining, without control; one whose power of deciding
      and governing is not limited.
      [1913 Webster]

            For Jove is arbiter of both to man.   --Cowper.
      [1913 Webster]

   Syn: Arbitrator; umpire; director; referee; controller;
        ruler; governor.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arbiter \Ar"bi*ter\, v. t.
   To act as arbiter between. [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

arbiter
    n 1: someone with the power to settle matters at will; "she was
         the final arbiter on all matters of fashion" [syn:
         arbiter, supreme authority]
    2: 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:

37 Moby Thesaurus words for "arbiter":
   JP, Justice, amateur, arbiter elegantiarum, arbiter of taste,
   arbitrator, authority, bencher, bon vivant, cognoscente, collector,
   connaisseur, connoisseur, critic, dilettante, epicure, epicurean,
   expert, good judge, gourmand, gourmet, his honor, his lordship,
   his worship, impartial arbitrator, indicator, judge, justice,
   magistrate, maven, moderator, referee, refined palate, third party,
   umpire, unbiased observer, virtuoso



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ARBITER. One who, decides without any control. A judge with the most
extensive arbitrary powers; an arbitrator.