Document Type Definition
Optional component of an XML document.
DTD is like a database schema: it defines and names the elements that can be used in the document, the order in which the elements can appear, the element attributes that can be used, and other document features.
To use a particular XML application, you usually include its DTD in your XML document. Having it in the document restricts the elements and structure that you can use so that your document is forced to conform to the application standard.