DTD: (THE DOCUMENT TYPE DEFINITION)
The first method used to provide this definition was the DTD, or Document Type Definition. This defines the elements that may be included in your document, what attributes these elements have, and the ordering and nesting of the elements.
The DTD is declared in a DOCTYPE declaration beneath the XML declaration contained within an XML document:
Inline Definition:
<?xml version="1.0"?>
<!DOCTYPE documentelement [definition]>External Definition:
<?xml version="1.0"?>
<!DOCTYPE documentelement SYSTEM "documentelement.dtd">