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
    n 1: a musical organization consisting of a group of
         instrumentalists including string players
    2: seating on the main floor in a theater

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "orchestra":
   German band, L, Philharmonic, R, acting area, apron, apron stage,
   audience, auditorium, auditory, backstage, balcony, band,
   band shell, bandstand, big band, board, box, box seat, brass,
   brass band, brass choir, brass quintet, brass section, brasses,
   bridge, callithumpian band, chamber orchestra, combo, concert band,
   congregation, coulisse, desks, dixieland band, dock, dress circle,
   dressing room, ensemble, fauteuil, flies, fly floor, fly gallery,
   forestage, gallery, gamelan orchestra, greenroom, grid, gridiron,
   groundling, group, house, jazz band, jug band, lightboard, loge,
   military band, nigger heaven, orchestra circle, orchestra pit,
   paradise, parquet, parquet circle, parterre, peanut gallery,
   performing area, philharmonic, pit, proscenium, proscenium boxes,
   proscenium stage, quartet, quintet, ragtime band,
   rock-and-roll group, sextet, shell, skiffle band, spectator, stage,
   stage left, stage right, stall, standing room, steel band,
   street band, string band, string choir, string orchestra,
   string quartet, strings, swing band, switchboard, symphony,
   symphony orchestra, the boards, theater, theatre stall, trio,
   waits, wings, woodwind, woodwind choir, woodwind quartet,
   woodwinds