There are mashups and then there are mashups, as I’m finding out. However, if you’re looking for a clean definition of a mashup, checkout WikiPedia:

“A mashup is a website or application that combines content from more than one source into an integrated experience.”

What has occurred is that public Web APIs, such as those stood up by guys like Google, are allowing savvy developers to take content and behavior from one SaaS, and mash it together with another, and thus form a new application custom-made to solve the business problems of the end user.