Excerpts from the SGML Standard

From ISO 8879:1986, Information Processing Text and Office Systems Standard Generalized Markup Language (SGML):

4.51 conforming SGML document
An SGML document that complies with all provisions of this International Standard.
4.102 document type
A class of documents having similar characteristics; for example, journal, article, technical manual, or memo.
4.105 document (type) definition
Rules, determined by an application, that apply SGML to the markup of documents of a particular type. A document type definition includes a formal specification, expressed in a document type delcaration, of the element types, element relationships and attributes, and references that can be represented by markup. It thereby defines the vocabulary of the markup for which SGML defines the syntax.