Document Type Definition
An optional, but powerful feature of XML that provides a formal set of rules to define a document structure
Defines the elements that may be used, and dictates where they may be applied in relation to each other; therefore specifies the document hierarchy and granularity
Comprises a set of declarations that define a document structure tree
Declarations stored either at the top of each document that must conform to the rules, or alternatively, and more usually, in separate data files, referred by a special instruction at the top of each document.