(1):
(n.) 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):
(n.) The score of a musical drama, either written or in print; a play set to music.
(3):
(n.) The house where operas are exhibited.
(4):
(pl.) of Opus