The extensible markup language (XML) is a standardized method for marking up texts and, optionally, for explicitly declaring the markup language used.
(By markup language here I mean a vocabulary (i.e., the set of elements that have meaning), and a grammar (i.e. how they relate to one another).
E.g., XHTML 1.1 is a markup language in which paragraph and header are part of the vocabulary, and the rules of the grammar say that paragraph is not allowed in the header part of a document.)