"A mashup is a web application that combines data from more than one source into a single integrated tool; a typical example is the use of cartographic data from Google Maps to add location information to real-estate data from Craigslist, thereby creating a new and distinct web service that was not originally envisaged by either source.

The term mashup originally referred to the practice in pop music (notably hip-hop) of producing a new song by mixing two or more existing pieces."

"Mashup." Wikipedia. WikiMedia Foundation. 7 Oct 2007  <http://en.wikipedia.org/wiki/Mashup_%28web_application_hybrid%29>

Resource

Mashups: The New Breed of Web App--Online article from IBM.  This articles explains what mashups are and gives examples of different types of mashups.

Programmable Web--A website that lists mashup applications and it is updated daily.