Markup languages are ways of annotating an electronic document. Usually markup will either specify how something should be displayed or what something means. The origin of the term is in typesetting, where proofs were marked up with instructions about their visual appearance, but the term then broadened to include the semantic perspective that we’re interested in here.

The names of the most popular languages usually end with Markup Language and so are abbreviated as something-ML: for example,

  • HTML – Hypertext Markup Language
  • KML – Keyhole Markup Language
  • MathML – Mathematical Markup Language
  • SGML – Standard Generalized Markup Language
  • XHTML – eXtensible Hypertext Markup Language
  • XML – eXtensible Markup Language