Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. The place in any public hall appropriated to a band of
      instrumental musicians.

   3. (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.

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

Source : WordNet®

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