opera vs theatre

opera and theatre both are nouns.

Word	Noun	Adjective	Verb	Adverb
opera	Yes	No	No	No
theatre	Yes	No	No	No

As nouns, theatre is a hypernym of opera; that is, theatre is a word with a broader meaning than opera:

opera: a building where musical dramas are performed
theatre: a building where theatrical performances or motion-picture shows can be presented

Other hypernyms of opera include house, theater.

opera (noun)	theatre (noun)
a building where musical dramas are performed	a building where theatrical performances or motion-picture shows can be presented
a drama set to music; consists of singing with orchestral accompaniment and an orchestral overture and interludes	the art of writing and producing plays
	a region in which active military operations are in progress