The term mash-up is most commonly used in the music business. There, it refers to the technique of taking elements of two (or more) pre-existing songs and combining those into a new track. It is related to concepts such as remixing and sampling.
More generally, a mash-up is any work of art created by combining existing elements from two or more sources. The video mash-up uses the same modus operandi, using footage from multiple sources (movies, television programs) to create a new montage. Video mash-ups are often made for comedic effect, but they have potential as a tool for essayist critique.
Take a look at some examples of mash-ups from our best practices section.