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]

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.