Xtensible Markup Language. A meta language used to describe the content and the structure of the data contained within. Object description language, sub-set of the SGML language (Standard Generalized Markup Language), a metalanguage used for document description. It is a marking language like HTML (also originating in SGML), and a document exchange and publishing standard. The markers used (unlike HTML) are also semantic indexes pointing on the content of the information described.