The Collaborative International Dictionary of English v.0.48:

Forum \Fo"rum\, n.; pl. E. Forums, L. Fora. [L.; akin to
   foris, foras, out of doors. See Foreign.]
   1. A market place or public place in Rome, where causes were
      judicially tried, and orations delivered to the people.
      [1913 Webster]

   2. A tribunal; a court; an assembly empowered to hear and
      decide causes.
      [1913 Webster]

            He [Lord Camden] was . . . more eminent in the
            senate than in the forum.             --Brougham.
      [1913 Webster]

WordNet (r) 3.0 (2006):

forum
    n 1: a public meeting or assembly for open discussion
    2: a public facility to meet for open discussion [syn: forum,
       assembly, meeting place]

Moby Thesaurus II by Grady Ward, 1.0:

153 Moby Thesaurus words for "forum":
   Areopagus, agora, airing, amphitheater, analysis, arena, assemblee,
   assembly, assignation, at home, athletic field, auditorium,
   background, ball, bear garden, board, bowl, boxing ring, brawl,
   bull ring, buzz session, campo, campus, canvas, canvassing, caucus,
   circus, cockpit, coliseum, colloquium, colosseum, commission,
   committee, conclave, concourse, conference, congregation, congress,
   consideration, conventicle, convention, convocation, council,
   course, curia, dance, date, debate, debating, deliberation,
   dialectic, dialogue, diet, discussion, discussion group,
   eisteddfod, examination, exchange of views, festivity, fete, field,
   floor, forgathering, gathering, get-together, ground, gym,
   gymnasium, hall, hippodrome, housewarming, inquisition,
   investigation, joint discussion, judicatory, judicature, judiciary,
   levee, lists, locale, logical analysis, logical discussion, market,
   market cross, marketplace, mart, mat, meet, meeting, milieu,
   open discussion, open forum, palaestra, panel, panel discussion,
   parade ground, party, piazza, pit, place, platform, plaza, plenum,
   precinct, prize ring, prom, public square, purlieu, quorum, rally,
   range, rap, rap session, reception, rendezvous, review, rialto,
   ring, round table, scene, scene of action, scenery, seance,
   seminar, session, setting, shindig, sit-in, site, sitting, soiree,
   sphere, square, squared circle, stadium, stage, stage set,
   stage setting, study, symposium, synod, terrain, the Inquisition,
   theater, tilting ground, tiltyard, town meeting, treatment,
   tribunal, turnout, ventilation, walk, wrestling ring



The Jargon File (version 4.4.7, 29 Dec 2003):

forum
 n.

    [Usenet, GEnie, CI$; pl. fora or forums] Any discussion group accessible
    through a dial-in BBS, a mailing list, or a newsgroup (see the
    network). A forum functions much like a bulletin board; users submit 
    postings for all to read and discussion ensues. Contrast real-time chat
    via talk mode or point-to-point personal email.


The Free On-line Dictionary of Computing (30 December 2018):

forum
fora
topic group

    (Plural "fora" or "forums") Any discussion group
   accessible through a dial-in BBS (e.g. GEnie, CI$), a
   mailing list, or a Usenet newsgroup (see network,
   the).  A forum functions much like a bulletin board; users
   submit postings for all to read and discussion ensues.

   Contrast real-time chat or point-to-point personal e-mail.

   [Jargon File]

   (1998-01-18)


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

FORUM. This term signifies jurisdiction, a court of justice, a tribunal.
     2. The French divide it into for exterieur, which is the authority
which human justice exercises on persons and property, to a greater or
lesser extent, according to the quality of those to whom it is entrusted;
and for interieur, which is the moral sense of justice which a correct
conscience dictates. Merlin, Repert. mot For.
     3. By forum res sitae is meant the tribunal which has authority to
decide respecting something in dispute, located within its jurisdiction;
therefore, if the matter in controversy is land, or other immovable
property, the judgment pronounced in the forum res sitae is held to be of
universal obligation, as to all matters of right and title on which it
professes to decide, in relation to such property. And the same principle
applies to all other cases of proceedings in rem, where the subject is
movable property, within the jurisdiction of the court pronouncing the
judgment. Story, Const. Laws, Sec. 532, 545, 551, 591, 592; Kaims on Eq. B.
3, c. 8, s. 4 1 Greenl. Ev. Sec. 541.