Extensible Markup Language, a simple and powerful standard for representing structured data. It is a markup language that defines a set of rules for encoding documents in a format which is both human readable and machine readable. It is a flexible language for creating common information formats and sharing both the format and content of data over the Internet and elsewhere. XML is a formatting language recommended by the World Wide Web Consortium (W3C).

Sources: https://en.wikipedia.org/wiki/XML ; ODH;  US OD