Merriam-Webster Collegiate Dictionary - riff

 I. noun  Etymology: probably by shortening & alteration from refrain  Date: 1935  1. an ostinato phrase (as in jazz) typically supporting a solo improvisation; also a piece based on such a phrase  2. a rapid energetic often improvised verbal outpouring; especially one that is part of a comic performance  3. a succinct usually witty comment  4. a distinct variation ; take a disturbing…~ on the Cinderella story — Daria Donnelly  II. intransitive verb  Date: 1948 to perform, deliver, or make use of a ~  III. verb  Etymology: short for ~le  Date: 1952 ~le, skim ~ pages