A Document Type Definition (DTD) is a set of markup declarations that define a document type for an SGML-family markup language (SGML, XML, HTML).

A DTD uses a terse formal syntax that declares precisely which elements and references may appear where in the document of the particular type, and what the elements’ contents and attributes are. A DTD can also declare entities which may be used in the instance document.

XML uses a subset of SGML DTD.

As of 2009^[update] newer XML namespace-aware schema languages (such as W3C XML Schema and ISO RELAX NG) have largely superseded DTDs. A namespace-aware version of DTDs is being developed as Part 9 of ISO DSDL[1]. DTDs persist in applications which need special publishing characters such as the XML and HTML Character Entity References, which were derived from the larger sets defined as part of the ISO SGML standard effort.

A DTD is associated with an XML or SGML document by means of a Document Type Declaration. The Document Type Declaration appears in the syntactic fragment doctypedecl near the start of an XML document. The declaration establishes that the document is an instance of the type defined by the referenced DTD.

Jennifer Widom is the chair of the Computer Science Department at Stanford University where she has worked on nontraditional data management. She is the Fletcher Jones Professor in Computer Science and a member of the National Academy of Engineering. She is the daughter of Harold Widom.

Widom has co-authored four academic textbooks, providing students with a beginning and advanced introduction to database systems. These books offer a comprehensive approach, focusing on database design, use, and implementation of applications and management systems. The course materials have been utilized at the junior, senior and graduate levels in Computer Science departments.