DTD : Document Type Definition.

It is a document that describes the structure of a Web page written in XML.

The description of the structure and the rules a document must satisfy for an SGML or XML document type. The DTD comprises the formal declaration of the elements that make up a document, their mutual coherence, meaning and documentation as drawn up for a document type (or document or information model).

You can store a DTD at the beginning of a document or externally in a separate file.