The Collaborative International Dictionary of English v.0.48:

Oratorio \Or`a*to"ri*o\, n. [It., fr. L. oratorius belonging to
   praying. See Orator, and cf. Oratory.]
   [1913 Webster]
   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.
      [1913 Webster]

   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.
         [1913 Webster]

   2. Performance or rendering of such a composition.
      [1913 Webster]

WordNet (r) 3.0 (2006):

oratorio
    n 1: a musical composition for voices and orchestra based on a
         religious text [syn: cantata, oratorio]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "oratorio":
   Negro spiritual, anthem, cantata, canticle, choral singing,
   chorale, chorus, church music, doxology, glee, gospel,
   gospel music, hymn, hymn-tune, hymnody, hymnology, introit,
   madrigal, madrigaletto, mass, motet, offertory, offertory sentence,
   paean, passion, prosodion, psalm, psalmody, recessional, requiem,
   requiem mass, sacred music, spiritual, unison, white spiritual