riff vs ostinato

riff and ostinato both are nouns.

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

As nouns, ostinato is a hypernym of riff; that is, ostinato is a word with a broader meaning than riff:

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

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

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