XPI stands for "XPInstall", a technology developed by the Mozilla Foundation for easily deploying their cross-platform software packages such as SeaMonkey, Thunderbird or Firefox.

The XPI format is similar to the ZIP compressed format with the only difference being that XPI files also include a RDF file which contains the resource description framework necessary for installing the software package.

XPI files may contain a compressed version of one of the applications above or a software extension designed for them.