What does XML stand for?

eXtensible Markup Language: some more precision, in reverse order:

XML is a language to define other languages: it is in fact a meta-language.

This informs us that the structuring elements of that language will be tags, like used in HTML: a tag name enclosed in brackets: <ATag>.

XML is not a language (which is by definition fixed) but a meta-language. XML is a set of rules, that a language must respect in order to be XML conformant. Therefore you can define your own mark-up language, and make it to evolve at your envy.