Extensible Markup Language [XML]

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(n.) A subset of Standard Generalized Markup Language (SGML) ISO 8879, upon which Hypertext Markup Language (HTML) is also based. The objective of XML is to enable generic SGML to be served, received, and processed on the web in the same way that HTML is handled. Unlike HTML it allows new tags to be defined and transmitted along with the content of a document.
The XML specification, recommended by the World Wide Web Consortium (W3C), describes a class of data objects called XML documents and describes how computer programs should process them. It is a restricted form of SGML and fully conforms to the SGML standard. Development was begun in 1996 by the XML Working Group of the W3C, and the standard was approved in 1998.
XML documents are comprised of storage units called entities, which contain either parsed or unparsed data. Parsed data consists of characters, some of which identify character data, and some of which identify markup. Markup defines a document’s storage layout and logical structure, and XML provides a mechanism to control these elements. An XML processor is a software module that is required to read XML documents and to provide access to their content and structure. The XML processor works in conjunction with an application. The XML specification describes the required behavior of an XML processor, how it must read XML data, and the information it must provide to the application.
The XML Version 1.0 specification provides the information needed to create XML documents and to process them on the Internet. Associated standards referenced in the specification are Unicode and ISO/IEC 10646 for characters, Internet RFC 1766 for language identification tags, ISO 639 for language name codes, and ISO 3166 for country name codes. The complete XML Version 1.0 specification may be found in the appendix on XML.