V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

DTD
       Document Type Definition (SGML, XML. HTML, DTD)


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)