What does the "X" in XML stand for?

Concept

XML is an acronym in which each letter expands to a word that describes the language. The letter that follows the role-word "Markup" and "Language" names the defining design property of the format: it lets authors define their own custom tags and document structures rather than being limited to a fixed, predefined tag set.

Application

Expanding the acronym letter by letter gives eXtensible Markup Language. The "X" stands for Extensible — meaning the vocabulary of tags can be extended by the user to suit any kind of data, which is exactly why XML is used as a general-purpose, self-describing data format.

Contrast

• Mixed: not part of the acronym; XML does support mixed content (text interleaved with elements), but that is a content model, not what the "X" means.

• Cross-platform: XML is portable across platforms, yet that is a benefit of the format, not the word the "X" abbreviates.

• Exclusive: the opposite of XML's intent — XML is open and extensible, not exclusive or restrictive.