- A way of depicting the logical structure or semantics of a document and providing instructions to computers on how to handle or display the contents of the file. HTML, XML and RDF are markup languages. Markup indicators are often called tags.
- A language that has codes for indicating layout and styling (such as boldface, italics, paragraphs, placement of graphics, etc.) within a text file. Widely used markup languages include SGML (Standard General Markup Language) and HTML (Hypertext Markup Language).