Source : Webster's Revised Unabridged Dictionary (1913)

Oratorio \Or`a*to"ri*o\, n. [It., fr. L. oratorius belonging to
   praying. See {Orator}, and cf. {Oratory}.]
   1. (Mus.) A more or less dramatic text or poem, founded on
      some Scripture nerrative, or great divine event,
      elaborately set to music, in recitative, arias, grand
      choruses, etc., to be sung with an orchestral
      accompaniment, but without action, scenery, or costume,
      although the oratorio grew out of the Mysteries and the
      Miracle and Passion plays, which were acted.

   Note: There are instances of secular and mythological
         subjects treated in the form of the oratorios, and
         called oratorios by their composers; as Haydn's
         ``Seasons,'' Handel's ``Semele,'' etc.

   2. Performance or rendering of such a composition.

Source : WordNet®

oratorio
     n : a musical composition for voices and orchestra based on a
         religious text [syn: {cantata}]