Good question! Hmmm.... just for fun, I'm going to agree with Haghj on the last definition, but put my own, completely baseless skew on riff and lick... a "riff" to me means a more complete phrase... so for instance a classic "riff" would be "Hair of the Dog"; a series of notes, like a solo, but repeated so that the riff becomes readily identifiable as a phrase of music. A "lick" to me is something that is not repeated, and is by itself incomplete... "oh, and in this part of the solo she plays this really cool lick... it goes like this..." So its a little musical unit all on its own, but its used in conjunction with other notes to complete a solo or phrase. That's just my own crack-pot take on it!