DTD stands for "Document Type Definition" as specified in the XML 1.x Recommendations of the W3C. DTDs define formal grammars for XML documents: which tags you can use and where you can use them. Validating XML processors apply this grammar to XML documents to determine whether they conform to these grammars, in which case the document is "valid".
DTD stands for "Document Type Declaration" as specified in the XML 1.x Recommendations of the W3C (see http://www.w3.org/TR/xml/).
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.
DTDs were also specified in SGML (ISO 8879:1986), the forbear to XML. SGML DTDs are more feature-rich than XML DTDs, allowing for example the specification of tag minimization features.
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