theater vs opera

theater and opera both are nouns.

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

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

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

theater (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