Document Type Definition

specification written in the Standard Generalized Markup Language and containing information about the format of a particular document, DTD (Computers)

Document Type Definition (DTD), defined slightly differently within the XML and SGML (the language XML was derived from) specifications, is one of several SGML and XML schema languages, and is also the term used to describe a document or portion thereof that is authored in the DTD language. A DTD is primarily used for the expression of a schema via a set of declarations that conform to a particular markup syntax and that describe a class, or type, of SGML or XML documents, in terms of constraints on the structure of those documents. A DTD may also declare constructs that are not always required to establish document structure, but that may affect the interpretation of some documents.
See more at Wikipedia.org...

<text, standard> (DTD) The definition of a document type in SGML or XML, consisting of a set of mark-up tags and their interpretation.
Docbook DTD home.
XML DTD Tutorial.
(2001-04-30)