The Standard Generalized Markup Language (SGML; ISO 8879:1986) is a standard for defining generalized markup languages for documents. ISO 8879 Annex A.1 defines generalized markup: 

Generalized markup is based on two postulates: 

HTML was theoretically an example of an SGML-based language until HTML 5, which admits that browsers cannot parse it as SGML (for compatibility reasons) and codifies exactly what they must do instead. 

DocBook SGML and LinuxDoc are better examples, as they were used almost exclusively with actual SGML tools. 

SGML is an ISO standard: "ISO 8879:1986 Information processing – Text and office systems – Standard Generalized Markup Language (SGML)", of which there are three versions: 

SGML is part of a trio of enabling ISO standards for electronic documents developed by ISO/IEC JTC1/SC34 (ISO/IEC Joint Technical Committee 1, Subcommittee 34 – Document description and processing languages) : 

SGML is supported by various technical reports, in particular 

SGML descended from IBM's Generalized Markup Language (GML), which Charles Goldfarb, Edward Mosher, and Raymond Lorie developed in the 1960s. Goldfarb, editor of the international standard, coined the “GML” term using their surname initials. Goldfarb also wrote the definitive work on SGML syntax in "The SGML Handbook". The syntax of SGML is closer to the COCOA format. As a document markup language, SGML was originally designed to enable the sharing of machine-readable large-project documents in government, law, and industry. Many such documents must remain readable for several decades—a long time in the information technology field. SGML also was extensively applied by the military, and the aerospace, technical reference, and industrial publishing industries. The advent of the XML profile has made SGML suitable for widespread application for small-scale, general-purpose use.
What does SGML stand for?
The Standard Generalized Markup Language (SGML; ISO 8879:1986)