Standard Generalized Markup Language (ISO 8879:1986) s an ISO-standard for document representation. SGML can be used for publishing in its broadest definition, ranging from single medium conventional publishing to multimedia data base publishing. SGML can also be used in office document processing when the benefits of human readability and interchange with publishing systems are required.
It is a basis for many popular markup-based languages used on the Web (HTML) or as a data storage/exchange format (XML, KML, …)
Source: ISO 8879:1986 specification