(or X11) A portable, network-transparent windowing system. The system consists of an X server dedicated to drawing on a display and receiving input events, and X clients that talk to the server in order to draw graphics or get input data. The X Window System is enormous but still the universal standard on most Unix systems.
XML
Extensible Markup Language. A widespread standard for creating document structure specifications. HTML and DocBook are XML-based formats. GNOME uses GNOME-XML to interface with XML.