Definition for XML DTD
From Biology Forums Dictionary
An XML DTD is a Document Type Declaration. A DTD is the first section of an XML document, while the document data itself makes up the second part of the document. The DTD begins with the DOCTYPE, and then details the name of the document and its structure. The data that is actually in the second part of the document may or may not conform to the DTD. If the document does conform to the DTD the document is called a type-valid XML document, and if the document does not conform to the DTD it is called a non-type-valid XML document. A document can be not-type-valid, and still be a good XML document.