WhatIs.com SGML defines SGML this way:
SGML (Standard Generalized Markup Language) is a standard for how to specify a document markup language or tag set. Such a specification is itself a document type definition (DTD). SGML is not in itself a document language, but a description of how to specify one. It is metadata.XML, like HTML, is a SGML-based language. XML is extensible because the markup symbols are unlimited and self-defining.
This was first published in April 2004