A mashup or bootleg is a song or composition created from the combination of the music from one song with the a cappella from another (also mash up and mash-up). These songs may be considered part of the bastard pop musical genre. "A mash-up is a song created out of pieces of two or more songs, usually by overlaying the vocal track of one song seamlessly over the music track of another...Mash-ups are incredible fun and a fascinating way to reexperience some of your favorite tunes."

In full swing at the start of the 21st century, mash ups have been described positively are "ultimate post-modern pop song[s]" or "'culture jamming in its purest form'" Creswell, Toby (2006). 1001 Songs: The Great Songs of All Time and the Artists, Stories and Secrets Behind Them, p.113. ISBN 1560259159. and "display an analytical impulse...we actually hear how these songs resonate with one another, comment upon and affect one another...in a way, the music speaks for itself" BaileyShea, Matt (2007). "Mignon: A New Recipe for Analysis and Recomposition", Music Theory Online Volume 13, Number 4, December 2007. . They have also been described negatively as "the logical extension of the sampling fever of the '80s taken to its dumbest extreme". Due to the questionable legal status of mash ups and little chance of profits "there's a real punk rock attitude attached to the movement" .