The Free On-line Dictionary of Computing (30 December 2018):

Document Type Definition
DTD

    (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 (http://oasis-open.org/docbook/).

   XML DTD Tutorial (http://xml101.com/dtd).

   (2001-04-30)