noun

1. a constantly repeated musical phrase used esp. as background for a soloist or as the basic theme of a final chorus: a term that originated in jazz music
2. Informal a short, incisive comment, passage, or scene, often one of several functioning as variations on a theme, in a speech, novel, film, etc.

Etymology: prob. altered < refrain

intransitive verb

1. to play a riff: a term that originated in jazz music
2. Informal to make use of a riff or riffs