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.

The etymology of this term almost certainly derives from its similar use in pop music where DJ's take the vocal track from one song and combine it with the instrumental track of another song resulting in an entirely new composition.

Many people are experimenting with mashups using eBay, Amazon, Google, and Yahoos APIs. The increased use and popularity of mashups has increased with the emergence of Web 2.0, which is characterized by active user participation and interaction.

Much like how blogs revolutionized online publishing, mashups are revolutionizing web development by giving creative power to the masses. Many mashups are relatively easy to design with minimal technical knowledge, and thus custom mashups are being designed by unlikely innovators, utilizing data in creative and unique ways. While there are several useful mashups, many are simple novelties or gimmicks, with minimal practical utility.

Most mashups interface with and utitilize information from established companies and web applications, occasionally without asking for permission. It is not uncommon for a mashup to display this information in a manner that the larger entity does not approve of, and a few scuffles have occurred on ethical grounds. Currently, mashups cannot be used for commercial purposes, and they are strictly non-profit, so the financial risk to the larger companies is minimal. Still, it is possible that if mashups prove to be a useful business tool or a viable market then corporate mashups will emerge as a paid-for service.

It is postulated that mashups are the beginning of the end for traditional operating systems. While operating systems currently serve as a collection of APIs for organizing and storing files and applications, it is quite possible that in a few years all APIs will exist entirely online, and that operating systems will be replaced by browsers.