1. [n]  someone chosen to judge and decide a disputed issue
2. [n]  someone with the power to settle matters at will; "she was the final arbiter on all matters of fashion"

ARBITER is a 7 letter word that starts with A.

1. \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.
   Note: In modern usage, arbitrator is the technical word.
   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.
            For Jove is arbiter of both to man.   --Cowper.
   Syn: Arbitrator; umpire; director; referee; controller;
        ruler; governor.
   

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