in full Extensible Markup Language.
SGML .
It incorporates features of HTML (e.g., hypertext linking), but is designed to overcome some of HTML's limitations. For example, it is designed to be extensible (i.e., designed to allow the creation of customized markup tags), which HTML is not. It is also designed to represent data by meaning rather than by layout (as it is represented in HTML). Like SGML, it is a metalanguage (a language for dealing with languages); it allows users to create a language tailored specifically to their needs.