XHTML
Extensible HyperText Markup Language. XHTML is a family of current and future document types and modules that reproduce, subset, and extend HTML 4 [HTML4]. XHTML family document types are XML based, and ultimately are designed to work in conjunction with XML-based user agents.

XML
Extensible Markup Language. A meta-language, abbreviated version of SGML, used to specify other document types used on the Web. Accepted as a format in 1998 to replace dependence on HTML extensions. MSIE 5.5 and Netscape 6 both support XML.

XML processor
A program that can XML documents and access their content.

XMODEM
A file transfer protocol. Rather slow.

XPFE
Cross Platform Front End. A suite of technologies used to create applications that will work and look the same on different computer operating systems. A widely used XPFE application is the Netscape web browser in version 7 and later. The primary technologies used in creating XPFE applications are Javascript, Cascading Style Sheets, and XUL.

XUL
Extensible User-interface Language. A markup language similar to HTML and based on XML. XUL used to define what the user interface will look like for a particular piece of software. XUL is used to define what buttons, scrollbars, text boxes, and other user-interface items will appear, but it is not used to define how those item will look (e.g. what color they are). The most widely used example of XUL use is probably in theMozilla web browser, where the entire user interface is defined using the XUL language.