a formal declaration that begins with the keyword DOCTYPE, followed by the name of the base element (also called the top or containing element) of that document, followed by the declaration subset which tells a system exactly what markup to expect.
A markup declaration that contains the formal specifications of a document type definition, shown as: DOCTYPE document_type_name optional_external_identifier [optional_document_type_declaration_subset ] The declaration invokes a DTD in an SGML document. The document instance of an SGML document must always be preceded by a document type declaration.
The DTD records the set of specific rules and properties that the XML must follow. These rules define, more particularly, the name and content of each tag as well as its context. This formalization of the elements can be used to check whether an XML document is in compliance (in which case, it is declared "valid"). The DTD may be included in the XML document (internal DTD) or in a separate document (external DTD). Note that the DTD is not mandatory.
The Document Type Declaration is a line at the beginning of a web page source that indicates the computer-readable language used in the page. This information ensures that browsers understand how to interpret the elements in the web page.
XML structural construct. Consists of markup code that indicates the grammar rules, or Document Type Definition (DTD), for the particular class of document. The document type declaration can also point to an external file that contains all or part of the DTD. It must appear following the XML declaration and preceding the document element. The syntax of the document type declaration is !DOCTYPE content.
From Extensible Markup Language (XML) 1.1 ( 2004-02-04) The XML document type declaration contains or points to markup declarations that provide a grammar for a class of documents. This grammar is known as a document type definition, or DTD. The document type declaration can point to an external subset (a special kind of external entity) containing markup declarations, or can contain the markup declarations directly in an internal subset, or can do both. The DTD for a document consists of both subsets taken together.
A Document Type Declaration, or DOCTYPE, is an instruction that associates a particular SGML or XML document (for example, a webpage) with a Document Type Definition (DTD) (for example, the formal definition of a particular version of HTML). In the serialized form of the document, it manifests as a short string of markup that conforms to a particular syntax.