XMLGUI is a KDE framework for designing the user interface of an application using XML, using the idea of actions.
Each action can be associated with various data including icons, explanatory text, and tooltips.
The interesting part to this design is that the actions are not inserted into the menus or toolbars by the programmer.
Instead, the programmer supplies an XML file, which describes the layout of the menu bar and toolbar.
The KDE XMLGUI is one in a long series of projects that have not managed to pin down the term for the resulting programming base.