Extensible Markup Language
n. (XML, abbr.)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.
NotesXML 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.
CitationsDoylen 2001, p. 3 XML documents are containers for information. Within the primary container may be information and more containers, which themselves may contain information and more containers. These named containers form neatly hierarchical structures, creating an incredibly flexible and remarkably powerful framework for storing and exchanging information of all kinds, from memos to database tables to poetry to program structures to invoices. XML documents may also include or reference sets of rules describing their structures, which applications may use to validate that documents conform to those rules.