The Collaborative International Dictionary of English v.0.48:

Quorum \Quo"rum\ (kw[=o]"r[u^]m), n. [L., of whom, gen. pl. of
   qui who, akin to E. who. See the Note below.]
   Such a number of the officers or members of any body as is
   competent by law or constitution to transact business; as, a
   quorum of the House of Representatives; a constitutional
   quorum was not present.
   [1913 Webster]

   Note: The term arose from the Latin words, Quorum aliquem
         vestrum . . . unum esse volumus (of whom we wish some
         one of you to be one), which were used in the
         commission formerly issued to justices of the peace in
         England, by which commission it was directed that no
         business of certain kinds should be done without the
         presence of one or more of certain justices specially
         designated. Justice of the peace and of the quorum
         designates a class of justices of the peace in some of
         the United States.
         [1913 Webster]

WordNet (r) 3.0 (2006):

quorum
    n 1: a gathering of the minimal number of members of an
         organization to conduct business

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "quorum":
   assemblee, assembly, assignation, at home, ball, brawl, caucus,
   colloquium, commission, committee, conclave, concourse,
   congregation, congress, conventicle, convention, convocation,
   council, dance, date, diet, eisteddfod, festivity, fete,
   forgathering, forum, gathering, get-together, housewarming, levee,
   meet, meeting, panel, party, plenum, prom, rally, reception,
   rendezvous, seance, session, shindig, sit-in, sitting, soiree,
   symposium, synod, turnout



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

QUORUM. Used substantively, quorum signifies the number of persons belonging
to a legislative assembly, a corporation, society, or other body, required
to transact business; there is a difference between an act done by a
definite number of persons, and one performed by an indefinite number: in
the first case a majority is required to constitute a quorum, unless the law
expressly directs that another number may make one; in the latter case any
number who may be present may act, the majority of those present having, as
in other cases, the right to act. 7 Cowen, 402; 9 B. & C. 648; Ang. on Corp.
28.1.
     2. Sometimes the law requires a greater number than a bare majority to
form a quorum, in such case no quorum is present until such a number
convene.
     3. When an authority is confided to several persons for a private
purpose, all must join in the act, unless otherwise authorized. 6 John. R.
38. Vide Authority, Majority; Plurality.



The Devil's Dictionary (1881-1906):

QUORUM, n.  A sufficient number of members of a deliberative body to
have their own way and their own way of having it.  In the United
States Senate a quorum consists of the chairman of the Committee on
Finance and a messenger from the White House; in the House of
Representatives, of the Speaker and the devil.