XML Namespace: Namespace (Computer Science), XML, Internationalized Resource Identifier, Uniform Identifier

Xpointer: XML, Software Framework, Xpath, Namespace (Computer Science)

XML Base: XML, W3C Recommendation, Uniform Resource Identifier, World Wide Web Consortium

XBEL: XML, Uniform Resource Identifier, Galeon, Konqueror, Arora (Browser)

XML Information Set: W3C Recommendation, XML, Namespace, Well Formed Document