Short for Standard Generalized Markup Language, SGML is an ISO standard that offers a general definition of a markup language. It lays out rules for what type of markup is valid, and how that markup should be parsed.
Not all markup languages conform to the SGML standard. For instance, HTML5 is not SGML-compatible.
Complete details of the standard may be viewed at https://www.iso.org/standard/16387.html.