Abbreviation for Extensible Markup Language. It a specification, similar to HTML, developed by the World Wide Web Consortium (W3C) for Web documents.  XML contains markup symbols (tags) to describe the contents of a page or file, but unlike HTML, the markup symbols are unlimited and self-defining (i.e. designers can create their own customized tags and tag definitions).  XML is a subset of SGML (Standard Generalized Markup Language).