(eXtensible Markup Language) The data tagging language of web services. XML is not so much a language as a standardized set of rules for adding structure to any form of data using a system of markup tags. Anyone can create their own markup vocabulary (called an XML Schema), and XML ensures that the structure will be intelligible to anyone else who consults the XML Schema document. More importantly, it enables XML-aware software to automatically manipulate the data without needing advance knowledge of the structure.