The term XTensions is a trademark of Quark, Inc. (www.quark.com) and describes a plugin for QuarkXPress, a popular page layout program.
An XTensions module is a little program that only runs inside QuarkXPress and enhances existing functionality or adds new functionality to the main program.
For example, XTensions can add new color models or full featured mathematical equation editor, allow this plugin to load additional file formats up to complex database publishing solutions that allow text on a layout to be pulled from a database and automatically create pages (based on templates).
QuarkXPress 1.0, which was first released in 1987, was—beside HyperCard—probably the first application ever to offer such an interface (or API) for plug-ins.
As the interface is available to third party developers, there are hundreds of XTensions available, both from commercial developers as well as open source XTensions.