XAML - Abbreviation for Extensible Application Markup Language (Microsoft Vista)

Xenu - A link-checking program.

XHTML - Abbreviation for Extensible Hypertext Markup Language.

XML - Abbreviation for Extensible Markup Language, a web programming language that allows web authors to define custom tags. Also file extension: filename.xml

XML Feeds - A form of paid inclusion where a search engine is "fed" information about pages via XML, rather than gathering that information through crawling actual pages. Marketers can pay to have their pages included in a spider based search index either annually per URL or on a CPC basis based on an XML document representing each page on the client site.

XMLP - XML Protocol

XMP -Abbreviation for Adobe Extensible Metadata Platform

XSL - Abbreviation for Extensible Scripting Language also file extension: filename.xsl

XSLT - XSL Transformations a language for transforming XML