document type definition

n. (DTD, abbr.) A set of rules that specify the structure of a document and the tags used to define that structure and that can be used to validate whether a document is well formed.


Originally specified in the SGML standard, DTDs are also specified in XML. A DTD may be included in a document header, or it may be a separate (external) resource referenced in the document header. A document type declaration is a statement in an XML document that names the particular document type definition used.


Hunter et al. 2000, p. 331 DTD refers only to the definition of a document type – not the DOCTYPE declaration that associates a DTD with an XML document instance.