From Science Europe Data Glossary
Definition: a markup language is a system for annotating a document in a way that is syntactically distinguishable from the text.
Explanation: The idea and terminology evolved from the "marking up" of paper manuscripts, i.e., the revision instructions by editors, traditionally written with a blue pencil on authors' manuscripts. In digital media this "blue pencil instruction text" was replaced by tags, that is, instructions are expressed directly by tags or "instruction text encapsulated by tags." Examples include typesetting instructions or structural markers such as XML tags. Markup instructs the software that displays the text to carry out appropriate actions, but is omitted from the version of the text that users see.