theatre vs opera

theatre and opera both are nouns.

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

As nouns, opera is a hyponym of theatre; that is, opera is a word with a more specific, narrower meaning than theatre:

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

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