Extensible Markup Language

n. (abbr. XML) a standard to promote sharing information over the internet by specifying ways to describe the information's semantic structure and to validate that the structure is well formed


XML is described as extensible because it is a metalanguage that allows the creation of tags to be used for semantic markup. The greatest value of XML can be realized through common vocabularies of tags, so that applications can use those vocabularies to understand documents from different sources.