A mashup (also mesh, mash up, mash-up, blend, bootleg  and bastard pop/rock) is a creative work, usually in a form of a song, created by blending two or more pre-recorded songs, usually by overlaying the vocal track of one song seamlessly over the instrumental track of another.
Scary eh? Even the videos kind of melt together.
And now the songs melt into each other...
Sometimes the videos and songs are surprisingly similar. Heck, its an industry, not a single artist.