DTD defines the document structure with a list of legal elements and attributes. It is both a definition and a standard as their are several standardized DTD definitions already defined for eBook use. It is also where Entity references are defined. The most common standard is XHTML that is used as part of the ePUB format. The definition pointer is often contained in a DOCTYPE statement.
An example form is shown in XHTML#Definition.
 For more information
- https://www.w3schools.com/xml/xml_dtd_intro.asp - A DTD tutorial
- http://en.wikipedia.org/wiki/Document_Type_Definition - WikiPedia article