Definition of oratorio noun from the Oxford Advanced Learner's Dictionary

oratorio

noun
ˌɒrəˈtɔːriəʊ
 
; ˌɔːrəˈtɔːrioʊ
 
; ˌɑːrəˈtɔːrioʊ
 
plural oratorios
 
a long piece of music for singers and an orchestra, usually based on a story from the Bible
compare cantata