XHTML
eXtensible HyperText Markup Language. Reformulation of HTML 4 according to XML rules. It is supported by HTML browsers and applications developed with XML.

XLL
eXtensible Linking Language. An XML extension used to insert links that can point directly to a specific object (image, title, word, etc.) into a page.

XML
eXtensible Markup Language. A re-defined version of SGML. It is seen as the successor of HTML. It enables the customising of tags which describes the layout and the very nature of data elements. It is very useful for sites maintaining large data volumes and on an intranet. Currently, only Microsoft Internet Explorer 5 and Netscape 6 offers support for XML.

XSL
eXtensible Style Sheet Language. A language for defining XML style sheets. It is on course to becoming a standard.