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.
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            For Jove is arbiter of both to man.   --Cowper.
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   Syn: Arbitrator; umpire; director; referee; controller;
        ruler; governor.
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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]