The Collaborative International Dictionary of English v.0.48:

Conclave \Con"clave\ (? or ?; 277), n. [F., fr. L. conclave a
   room that may locked up; con- + clavis key. See Clavicle.]
   1. The set of apartments within which the cardinals of the
      Roman Catholic Church are continuously secluded while
      engaged in choosing a pope.
      [1913 Webster]

   2. The body of cardinals shut up in the conclave for the
      election of a pope; hence, the body of cardinals.
      [1913 Webster]

            It was said a cardinal, by reason of his apparent
            likelihood to step into St. Peter's chair, that in
            two conclaves he went in pope and came out again
            cardinal.                             --South.
      [1913 Webster]

   3. A private meeting; a close or secret assembly.
      [1913 Webster]

            The verdicts pronounced by this conclave (Johnson's
            Club) on new books, were speedily known over all
            London.                               --Macaulay.
      [1913 Webster]

   To be in conclave, to be engaged in a secret meeting; --
      said of several, or a considerable number of, persons.
      [1913 Webster]

WordNet (r) 3.0 (2006):

conclave
    n 1: a confidential or secret meeting

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "conclave":
   Council of Nicaea, Council of Trent, Lateran Council,
   Vatican Council, assemblee, assembly, assignation, at home,
   audience, ball, bargaining, bargaining session, brawl, caucus,
   chapter, classis, colloquium, commission, committee, conciliarism,
   concourse, confab, confabulation, conference, confrontation,
   congregation, congress, consistory, constitutional convention,
   consultation, conventicle, convention, convocation, council,
   council fire, council of war, county convention, dance, date, diet,
   diocesan conference, discussion, ecclesiastical council,
   ecumenical council, eisteddfod, exchange of views,
   eyeball-to-eyeball encounter, festivity, fete, forgathering, forum,
   gathering, get-together, high-level talk, housewarming, huddle,
   interchange of views, interview, levee, meet, meeting,
   national convention, negotiations, news conference,
   nominating convention, palaver, panel, parley,
   parochial church council, parochial council, party,
   plenary council, plenum, political convention, pourparler, powwow,
   preliminary convention, presbytery, press conference, prom,
   quadrennial circus, quorum, rally, reception, rendezvous, seance,
   session, shindig, sit-in, sitting, soiree, state convention,
   summit, summit conference, summitry, symposium, synod, turnout,
   vestry



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

CONCLAVE. An assembly of cardinals for the purpose of electing a pope; the
place where the assembly is held is also called a conclave. It derives this
name from the fact that all the windows and doors are looked, with the
exception of a single panel, which admits a gloomy light.