A search for a definition of the currently much- (some say over-) used word mashup takes you to this wikipedia disambiguation page (i.e. it has more than one meaning) which then leads to the following:
Mashup (music) redirects you to this definition:
Bastard pop is a musical genre which, in its purest form, consists of the combination (usually by digital means) of the music from one song with the a cappella from another. Typically, the music and vocals belong to completely different genres. At their best, bastard pop songs strive for musical epiphanies that add up to considerably more than the sum of their parts.
Mashup (web application hybrid) is defined as:
A mashup is a website or web application that seamlessly combines content from more than one source into an integrated experience.
Content used in mashups is typically sourced from a third party via a public interface or API. Other methods of sourcing content for mashups include Web feeds (e.g. RSS or Atom) and JavaScript includes.
The etymology of this term almost certainly derives from its similar use in pop music.
Many people are experimenting with mashups using eBay, Amazon, Google, and Yahoos APIs.