In music, a medley is a piece composed from parts of existing pieces, usually three, played one after another, sometimes overlapping. They are common in popular music, and most medleys are songs rather than instrumental. A medley which is a remixed series is called a megamix, often done with tracks for a single artist, or for popular songs from a given year or genre.
http://en.wikipedia.org/wiki/Medley_(music)
http://en.wikipedia.org/wiki/Medley_(music)
