A markup language is a system for annotating a document in a way that is syntactically distinguishable from 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, meaning 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.
HyperText Markup Language (HTML), one of the document formats of the World Wide Web, is an instance 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.