serious music vs opera

serious music and opera both are nouns.

Word	Noun	Adjective	Verb	Adverb
serious music	Yes	No	No	No
opera	Yes	No	No	No

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

serious music: traditional genre of music conforming to an established form and appealing to critical interest and developed musical taste
opera: a drama set to music; consists of singing with orchestral accompaniment and an orchestral overture and interludes

Other hyponyms of serious music include chamber music, cantata, oratorio, concerto, fugue, rondeau, rondo.

serious music (noun)	opera (noun)
traditional genre of music conforming to an established form and appealing to critical interest and developed musical taste	a building where musical dramas are performed
	a drama set to music; consists of singing with orchestral accompaniment and an orchestral overture and interludes