serious music vs rondo

serious music and rondo both are nouns.

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

As nouns, rondo is a hyponym of serious music; that is, rondo 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
rondo: a musical form that is often the last movement of a sonata

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

serious music (noun)	rondo (noun)
traditional genre of music conforming to an established form and appealing to critical interest and developed musical taste	a musical form that is often the last movement of a sonata