Mashups

Summary 

Generally a mashup is some sort of web application that links two or more formerly unrelated data sources together, usually by using some sort of 3rd-party code and/or javascript, etc.

Examples 

A simple, direct example of this is http://www.housingmaps.com/.

This website takes two distinct, completely unrelated data sources (Google Maps and Craig's List) and mixes them together with (mostly) javascript.

Further Consideration

http://davetroy.blogspot.com/2008/02/mashup-is-dead.html