The Collaborative International Dictionary of English v.0.48:

Tribunal \Tri*bu"nal\, n. [L. tribunal, fr. tribunus a tribune
   who administered justice: cf. F. tribunal. See Tribune.]
   1. The seat of a judge; the bench on which a judge and his
      associates sit for administering justice.
      [1913 Webster]

   2. Hence, a court or forum; as, the House of Lords, in
      England, is the highest tribunal in the kingdom.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tribunal \Tri`bu*nal"\, n. [Sp.]
   In villages of the Philippine Islands, a kind of townhall. At
   the tribunal the head men of the village met to transact
   business, prisoners were confined, and troops and travelers
   were often quartered.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

tribunal
    n 1: an assembly (including one or more judges) to conduct
         judicial business [syn: court, tribunal, judicature]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "tribunal":
   British Cabinet, Sanhedrin, US Cabinet, advisory body, assembly,
   association, balcony, bar, bench, board, body of advisers,
   borough council, brain trust, cabinet, camarilla, catafalque,
   chamber, city council, common council, conference, congress,
   consultative assembly, council, council fire, council of ministers,
   council of state, council of war, county council, court,
   court of justice, curial, dais, deliberative assembly, diet,
   directory, divan, emplacement, estrade, floor, gallery, heliport,
   hustings, judicial, judiciary, junta, kitchen cabinet, landing,
   landing pad, landing stage, launching pad, legislature,
   parish council, platform, podium, privy council, pulpit, rostrum,
   soapbox, soviet, staff, stage, step terrace, stump, syndicate,
   synod, terrace, tribune



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

TRIBUNAL. The seat of a judge; the place where he administers justice; but
by this term is more usually understood the whole body of judges who compose
a jurisdiction sometimes it is taken for the jurisdiction which they
exercise.
     2. This term is Latin, and derives its origin from the elevated seat
where the tribunes administered justice.