What does DOCTYPE mean?

DOCTYPE or Document Type Declaration is a type of instruction which usually works in association with particular SGML or XML documents basically. Let us take an example to understand it more thoroughly, for example, A Web page with a document type definition i.e. DTD is the best to understand. In a well serialized and a proper form of the document, It manifests and also the contribution of it is a lot as a short string of markup that usually conforms to a particular syntax.