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

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.