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?實例:
<!ENTITY?writer?"Donald?Duck.">
<!ENTITY?copyright?"Copyright?runoob.com">
XML?實例：
<author>&writer;©right;</author>