The Collaborative International Dictionary of English v.0.48:

Orchestra \Or"ches*tra\, n. [L. orchestra, Gr. ?, orig., the
   place for the chorus of dancers, from ? to dance: cf. F.
   orchestre.]
   1. The space in a theater between the stage and the audience;
      -- originally appropriated by the Greeks to the chorus and
      its evolutions, afterward by the Romans to persons of
      distinction, and by the moderns to a band of instrumental
      musicians. Now commonly called orchestra pit, to
      distinguish it from the section of the main floor occupied
      by spectators.
      [1913 Webster +PJC]

   2. The space in the main floor of a theater in which the
      audience sits; also, the forward spectator section of the
      main floor, in distinction from the parterre, which is
      the rear section of the main floor.
      [PJC]

   3. The place in any public hall appropriated to a band of
      instrumental musicians.
      [1913 Webster]

   4. (Mus.)
      (a) Loosely: A band of instrumental musicians performing
          in a theater, concert hall, or other place of public
          amusement.
      (b) Strictly: A band suitable for the performance of
          symphonies, overtures, etc., as well as for the
          accompaniment of operas, oratorios, cantatas, masses,
          and the like, or of vocal and instrumental solos.
      (c) A band composed, for the largest part, of players of
          the various viol instruments, many of each kind,
          together with a proper complement of wind instruments
          of wood and brass; -- as distinguished from a military
          or street band of players on wind instruments, and
          from an assemblage of solo players for the rendering
          of concerted pieces, such as septets, octets, and the
          like.
          [1913 Webster]

   5. (Mus.) The instruments employed by a full band,
      collectively; as, an orchestra of forty stringed
      instruments, with proper complement of wind instruments.
      [1913 Webster]

WordNet (r) 3.0 (2006):

orchestra pit
    n 1: lowered area in front of a stage where an orchestra
         accompanies the performers [syn: orchestra pit, pit]