[ ˌɒrə'tɔ:rɪəʊ ]
■ noun ( plural ~s ) a large-scale semi-dramatic musical work for orchestra and voices on a sacred theme, performed without costume, scenery, or action.
Origin
Ital., from eccles. L. oratorium 'oratory', from the musical services held in the church of the Oratory of St Philip Neri in Rome.