XML (Extensible Markup Language) is a set of rules for encoding documents electronically. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards.

XML's design goals emphasize simplicity, generality, and usability over the Internet.It is a textual data format, with strong support viaUnicode for the languages of the world. Although XML's design focuses on documents, it is widely used for the representation of arbitrary data structures, for example in web services.

There are many programming interfaces that software developers may use to access XML data, and several schema systems designed to aid in the definition of XML-based languages.

As of 2009, hundreds of XML-based languages have been developed,including RSS, Atom, SOAP, and XHTML. XML-based formats have become the default for most office-productivity tools, including Microsoft Office (Office Open XML), OpenOffice.org (OpenDocument), and Apple's iWork.