|SGML Document Type Definition (DTD) file
DTD is a file extension for a Document Type Definition (DTD) file format used in SGML, XML, HTML or XHTML to identify what the variables (or markup) are that format a document. In the case of a text document, for example, markup would specify separating paragraphs, identifying topic headings, and so forth and how each is to be processed.
Learn more about DTD files:
What is the intent of the DTD? Expert response from Sean McGrath at Search SOA.
Search for answers to DTD-related questions at IT Knowledge Exchange.