Webster's Dictionary 
(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
The Nuttall Encyclopedia 
A drama set to music and acted and sung to the accompaniment of a full orchestra, of which there are several kinds according as they are grave, comic, or romantic.