opera vs theater

opera and theater both are nouns.

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

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

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

Other hypernyms of opera include house, theatre.

opera (noun)	theater (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