A 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 manuscripts, i.e., the revision instructions by editors, traditionally written with a blue pencil on authors' manuscripts. Examples are typesetting instructions such as those found in troff and LaTeX, or structural markers such as XML tags. Markup is typically omitted from the version of the text that is displayed for end-user consumption. Some markup languages, such as HTML, have presentation semantics, meaning that their specification prescribes how the structured data are to be presented, but other markup languages, like XML, have no predefined semantics.
A well-known example of a markup language in widespread use today is HyperText Markup Language (HTML), one of the document formats of the World Wide Web. HTML, which is an instance of SGML (though, strictly, it does not comply with all the rules of SGML), follows many of the markup conventions used in the publishing industry in the communication of printed work between authors, editors, and printers.