ostinato vs riff

ostinato and riff both are nouns.

Word	Noun	Adjective	Verb	Adverb
ostinato	Yes	No	No	No
riff	Yes	No	Yes	No

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

ostinato: a musical phrase repeated over and over during a composition
riff: a jazz ostinato; usually provides a background for a solo improvisation

Other hyponyms of ostinato include ground bass.

ostinato (noun)	riff (noun)
a musical phrase repeated over and over during a composition	a jazz ostinato; usually provides a background for a solo improvisation

ostinato (verb)	riff (verb)
	play riffs
	look through a book or other written material