The current online Merriam Webster gives 4 definitions for riff as a noun. So 45 years and the Atlantic Ocean does make a difference. The relevant one is similar but different:

an ostinato phrase (as in jazz) typically supporting a solo improvisation

[There was an interesting piece in the New Yorker recently: Is the Dictionary Done For? (Paywalled, but maybe somebody here is a subscriber and can share a link.]