An unofficial track that combines or reworks existing songs, often the vocal of one over the instrumental of another.
A mashup, or bootleg, is an unofficial blend that combines elements of existing tracks, classically an acapella of one song over the instrumental of another. Bootlegs are usually shared rather than commercially released.
Why it matters
Mashups create unique moments only you can play. They are also why acapellas and instrumentals are worth keeping organized.

