Wikipedia defines 'Riff'

As a jazz saxophonist, of little repute, I enjoy finding a riff in a solo and running with it. To me it provides context and can make the solo ride just that much more fun. In jazz, solo's are rarely scripted or written out, so every time I play a solo, it's like finding a new path. But if I have a feel for the riff, then there is much joy in jazz land, at least for me. And using the riff I can create consistently interesting solos.

Wikipedia defines riff thusly: In music, a riff is an ostinato figure: a repeated chord progression, pattern or melodic figure, often played by the rhythm section instruments, that forms the basis or accompaniment of a rock music or jazz composition. They can be as simple as repeated saxophone honking an entire chorus on a single exciting note by Big Jay McNeely or as complex as the head arrangements played by the Count Basie Orchestra.

David Brackett (1999) defines them as, "short melodic phrases," while Richard Middleton (1999) defines them as, "short rhythmic, melodic, or harmonic figures repeated to form a structural framework." Though Rikky Rooksby (2002, p.6-7) admits that there is no "water-tight" definition, he gives a "working description" for riffs in rock: "A riff is a short, repeated, memorable musical phrase, often pitched low on the guitar, which focuses much of the energy and excitement of a rock song."

Riffs are often used (and re-used) to create full compositions. The "Night Train" riff was first used in Duke Ellington's "Happy-Go-Lucky Local", which Ellington had recycled from Johnny Hodges earlier "That's the Blues, Old Man". The riff from Charlie Parker's bebop number "Now's the Time" (1945) re-emerged four years later as the R&B dance hit, "The Hucklebuck", while Glenn Miller's "In the Mood" had an earlier life as Wingy Manone's "Tar Paper Stomp". (All these songs use twelve bar blues riffs. It is probable that these riffs precede the examples given.)