In reference to computers DTD stands for document type definition. It is defined as,

"a set of markup declarations that define a document type for SGML-family markup languages (SGML, XML, HTML). DTDs use 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."

- wikipedia

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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.