1. In computing, Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. The W3C's XML 1.0 Specification and several other related specifications all of them free open standards define XML.
The design goals of XML emphasize simplicity, generality, and usability across the Internet.It is a textual data format with strong support via Unicode for different human languages. Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary data structures such as those used in web services.
2. XML, in full extensible markup language, a document formatting language used for some World Wide Web pages. XML began to be developed in the 1990s because HTML (hypertext markup language), the basic format for Web pages, does not allow the definition of new text elements; that is, it is not extensible. XML is a simplified form of SGML (standard generalized markup language) intended for do are published on the Web.
https://en.wikipedia.org/wiki/XML (1.); https://www.britannica.com/technology/XML (2.)