Entities are used to define shortcuts to special characters.
Entities can be declared internal or external.
An Internal Entity Declaration
Syntax
<!ENTITY entity-name "entity-value">
Example
DTD Example:
<!ENTITY writer "Donald Duck.">
<!ENTITY copyright "Copyright w3ii.">
XML example:
<author>&writer;©right;</author>
Note: An entity has three parts: an ampersand (&), an entity name, and a semicolon (;).
An External Entity Declaration
Syntax
<!ENTITY entity-name SYSTEM "URI/URL">
Example
DTD Example:
<!ENTITY writer SYSTEM "http://www.w3ii.com/entities.dtd">
<!ENTITY copyright SYSTEM "http://www.w3ii.com/entities.dtd">
XML example:
<author>&writer;©right;</author>