Source : Webster's Revised Unabridged Dictionary (1913)

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

   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.)

Opera \Op"er*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, arials, choruses, duets, trios, etc., with
      orchestral accompaniment, preludes, and interludes,
      together with appropriate costumes, scenery, and action; a
      lyric drama.

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

   3. The house where operas are exhibited.

   {Op['e]ra 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.

   {Op['e]ra comique} [F.], comic or humorous opera.

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

   {Opera girl} (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.

Source : WordNet®

opera
     n 1: a drama set to music; consists of singing with orchestral
          accompaniment and an orchestral overture and interludes
     2: theater where opera is performed [syn: {opera house}]