Document type definitions (DTDs) define legal building blocks for XML documents. It uses a series of legal elements to define the structure of the document.
A DTD can be declared in an XML document as a line or as an external reference.
DTD - Entity syntax
Entities are variables used to define shortcuts that reference normal text or special characters.
An entity reference is a reference to an entity.
Entities can be declared internally or externally.
DTD - Entity example
DTD 实例: <!ENTITY writer "Donald Duck."> <!ENTITY copyright "Copyright runoob.com"> XML 实例： <author>&writer;©right;</author>