A (document) markup language is a modern system for annotating a document in a way that is syntactically distinguishable from the text. 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 such as those found in troff, TeX and LaTeX, 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.
Some markup languages, such as the widely used HTML, have pre-defined presentation semantics meaning that their specification prescribes how to present the structured data. Others, such as XML, do not.
HyperText Markup Language (HTML), one of the document formats of the World Wide Web, is an instance of SGML (though, strictly, it does not comply with all the rules of SGML), and follows many of the markup conventions used in the publishing industry in the communication of printed work between authors, editors, and printers.