The Collaborative International Dictionary of English v.0.48:

Umpire \Um"pire\, v. t. [imp. & p. p. Umpired; p. pr. & vb. n.
   Umpiring.]
   1. To decide as umpire; to arbitrate; to settle, as a
      dispute.
      [1913 Webster]

            Judges appointed to umpire the matter in contest
            between them, and to decide where the right lies.
                                                  --South.
      [1913 Webster]

   2. To perform the duties of umpire in or for; as, to umpire a
      game. [Colloq.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Umpire \Um"pire\, v. i.
   To act as umpire or arbitrator.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Umpire \Um"pire\, n. [OE. nompere, nounpere (also impier, fr. F.
   impair uneven), fr. OF. nomper uneven, F. non-pair; hence the
   meaning, uneven, i. e., third person; non not + OF. per even,
   equal, peer, F. pair; cf. L. impar uneven, unequal. See
   Non-, and Peer, n.]
   [1913 Webster]
   1. A person to whose sole decision a controversy or question
      between parties is referred; especially, one chosen to see
      that the rules of a game, as cricket, baseball, or the
      like, are strictly observed.
      [1913 Webster]

            A man, in questions of this kind, is able to be a
            skillful umpire between himself and others.
                                                  --Barrow.
      [1913 Webster]

   2. (Law) A third person, who is to decide a controversy or
      question submitted to arbitrators in case of their
      disagreement. --Blackstone.
      [1913 Webster]

   Syn: Judge; arbitrator; referee. See Judge.
        [1913 Webster]

WordNet (r) 3.0 (2006):

umpire
    n 1: an official at a baseball game [syn: umpire, ump]
    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]
    v 1: be a referee or umpire in a sports competition [syn:
         referee, umpire]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "umpire":
   JP, Justice, act between, adjudge, adjudicate, adjudicator,
   arbiter, arbitrate, arbitrator, bargain, bencher, critic,
   go between, hear, his honor, his lordship, his worship, hold court,
   hold the scales, impartial arbitrator, indicator, intercede,
   intermediate, interpose, intervene, judge, judger, justice,
   magistrate, make terms, mediate, meet halfway, moderate, moderator,
   negotiate, official, officiate, referee, represent,
   sit in judgment, step in, third party, treat with, try, ump,
   unbiased observer



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

UMPIRE. A person selected by two or more arbitrators. When they are
authorize to do so by the submission of the parties, and they cannot agree
as to the subject-matter referred to them, whose duty it is to decide the
matter in dispute. Sometimes the term is applied to a single arbitrator,
selected by the parties themselves. Kyd on Awards, 6, 75, 77 Caldw. on Arb.
38; Dane's Ab. Index, h.t.; 3 Vin. Ab. 93; Com. Dig. Arbitrament, F; 4 Dall.
271, 432; 4 Sco. N. S. 378; Bouv. Inst. Index, h.t.