opera vs rock opera

opera and rock opera both are nouns.

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

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

opera: a drama set to music; consists of singing with orchestral accompaniment and an orchestral overture and interludes
rock opera: an opera with rock music

Other hyponyms of opera include bouffe, comic opera, opera bouffe, opera comique, grand opera, musical drama.

opera (noun)	rock opera (noun)
a building where musical dramas are performed	an opera with rock music
a drama set to music; consists of singing with orchestral accompaniment and an orchestral overture and interludes