The Collaborative International Dictionary of English v.0.48:

Opus \O"pus\, n.; pl. Opera. [L. See Opera.]
   A work; specif. (Mus.), a musical composition.
   [1913 Webster]

   Note: Each composition, or set of pieces, as the composer may
         choose, is called an opus, and they are numbered in the
         order of their issue. (Often abbrev. to op.)
         [1913 Webster]

   Opus incertum. [L.] (Arch.) See under Incertum.
      [1913 Webster] Opuscle

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
    n 1: a drama set to music; consists of singing with orchestral
         accompaniment and an orchestral overture and interludes
    2: a commercial browser
    3: a building where musical dramas are performed [syn: opera,
       opera house]

Moby Thesaurus II by Grady Ward, 1.0:

200 Moby Thesaurus words for "opera":
   Broadway musical, Elizabethan theater, Globe Theatre,
   Grand Guignol, Greek theater, Passion play, Singspiel, Tom show,
   amphitheater, antimasque, arena theater, arrangement, artifact,
   audience success, auditorium, ballad opera, ballet,
   ballet divertissement, bomb, brainchild, broadcast drama,
   burlesque show, cabaret, charade, child, chorus show,
   circle theater, cliff hanger, closet drama, club, coinage,
   comedy ballet, comedy drama, comic opera, composition,
   concert hall, concoction, copy, creation, creature,
   critical success, crowning achievement, dance drama,
   daytime serial, dialogue, distillation, documentary drama, draft,
   drama, dramalogue, dramatic play, dramatic series, duodrama,
   duologue, edition, effect, end product, epic theater, essence,
   experimental theater, extract, extravaganza, failure, flop, fruit,
   gasser, giveaway, grand opera, hall, handiwork, happening, hit,
   hit show, house, hymnal, hymnbook, improvisational drama,
   instrumental score, invention, issue, legitimate drama, libretto,
   light opera, little theater, lute tablature, lyric drama,
   manufacture, masque, masterpiece, masterwork, melodrama, minstrel,
   minstrel show, mintage, miracle, miracle play, monodrama,
   monologue, morality, morality play, music, music drama, music hall,
   music paper, music roll, musical, musical comedy, musical notation,
   musical revue, musical score, musical stage, musical theater,
   mystery, mystery play, new mintage, night spot, nightclub,
   notation, offspring, opera ballet, opera bouffe, opera buffa,
   opera house, opera score, operetta, opus, opuscule,
   orchestral score, origination, outcome, outdoor theater, outgrowth,
   pageant, panel show, pantomime, part, pastoral, pastoral drama,
   piano score, piece, play, playhouse, playlet, problem play,
   product, production, psychodrama, quiz show, radio drama, result,
   review, revue, score, sensational play, serial, sheet music,
   short score, show, showboat, sitcom, situation comedy, sketch,
   skit, soap, soap opera, sociodrama, song-and-dance act, song-play,
   songbook, songster, spectacle, stage play, stage show,
   straight drama, success, suspense drama, tablature, tableau,
   tableau vivant, talk show, teleplay, television drama,
   television play, text, theater, theater of cruelty,
   theater-in-the-round, theatron, total theater, transcript,
   transcription, variety show, vaudeville, vaudeville show, vehicle,
   version, vocal score, word-of-mouth success, work, written music



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

OPERA
       Open PLC European Research Alliance (org., PLC, Europe)


The Devil's Dictionary (1881-1906):

OPERA, n.  A play representing life in another world, whose
inhabitants have no speech but song, no motions but gestures and no
postures but attitudes.  All acting is simulation, and the word
_simulation_ is from _simia_, an ape; but in opera the actor takes for
his model _Simia audibilis_ (or _Pithecanthropos stentor_) -- the ape
that howls.

    The actor apes a man -- at least in shape;
    The opera performer apes and ape.