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I found this definition of a mash up from Vinton Cerf (you know the guy who invented the Internet before Al Gore).  Web Application Hybird

"We know we don’t have a corner on creativity. There are creative people all around the world, hundreds of millions of them, and they are going to think of things to do with our basic platform that we didn’t think of. So the mashup stuff is a wonderful way of allowing people to find new ways of applying the basic infrastructures we’re propagating. This will turn out to be a major source of ideas for applying Google-based technology to a variety of applications.”