X-Axis – Horizontal plane of a two-dimensional plot. Or one of the two horizontal planes of a three-dimensional plot where it will be at a right-angle to the y-axis.
XHTML – Abbreviation for Extensible Hypertext Mark-up Language and is a hybrid of XML and HTML. Web pages designed in XHTML should look the same across all platforms. XHTML is a reformation of HTML as an XML vocabulary.
XML – Abbreviation for Extensible Mark-up Language. XML A language specification from the W3C that allows users to develop their own markup languages (often called vocabularies), and format their documents using stylesheets to be presented on a browser if desired.