The Collaborative International Dictionary of English v.0.48:

Opera \Op"er*a\ ([o^]p"[~e]r*[.a]), n. [It., fr. opera work,
   composition, opposed to an improvisation, fr. L. opera pains,
   work, fr. opus, operis, work, labor: cf. F. op['e]ra. See
   Operate.]
   1. A drama, either tragic or comic, of which music forms an
      essential part; a drama wholly or mostly sung, consisting
      of recitative, arias, choruses, duets, trios, etc., with
      orchestral accompaniment, preludes, and interludes,
      together with appropriate costumes, scenery, and action; a
      lyric drama.
      [1913 Webster]

   2. The score of a musical drama, either written or in print;
      a play set to music.
      [1913 Webster]

   3. The house where operas are exhibited.
      [1913 Webster]

   Opera bouffe [F. op['e]ra opera + bouffe comic, It. buffo],
   Opera buffa [It.], light, farcical, burlesque opera.

   Opera box, a partially inclosed portion of the auditorium
      of an opera house for the use of a small private party.

   Opera comique [F.], comic or humorous opera.

   Opera flannel, a light flannel, highly finished. --Knight.


   Opera girl or Opera girls (Bot.), an East Indian plant
      (Mantisia saltatoria) of the Ginger family, sometimes
      seen in hothouses. It has curious flowers which have some
      resemblance to a ballet dancer, whence the popular name.
      Called also dancing girls.

   Opera glass, a short telescope with concave eye lenses of
      low power, usually made double, that is, with a tube and
      set of glasses for each eye; a lorgnette; -- so called
      because adapted for use at the opera, theater, etc.

   Opera hat, a gentleman's folding hat.

   Opera house, specifically, a theater devoted to the
      performance of operas.

   Opera seria [It.], serious or tragic opera; grand opera.
      [1913 Webster]

WordNet (r) 3.0 (2006):

opera house
    n 1: a building where musical dramas are performed [syn:
         opera, opera house]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "opera house":
   Elizabethan theater, Globe Theatre, Greek theater, amphitheater,
   arena, arena theater, assembly hall, auditorium, cabaret, chapel,
   circle theater, club, concert hall, convention hall, dance hall,
   exhibition hall, gallery, hall, house, lecture hall,
   little theater, meetinghouse, music hall, night spot, nightclub,
   opera, outdoor theater, playhouse, showboat, stadium, theater,
   theater-in-the-round, theatron