An entity is essentially a macro that allows a short name to be associated with replacement text. Parameter entities define replacement text used in DTD declarations. Syntactically, a parameter entity is distinguished by using the percent (%) symbol. Its general form is shown here:
<!ENTITY % name "replacement text">
It is used in DTDs as follows:
%name;
Parameter entities are a convenient way to define commonly occurring pieces of a DTD so that changes only need to be made in one place. We see in XHTML a parameter entity to define the core attributes common to most elements.
<!ENTITY % coreattrs "id ID #IMPLIED class CDATA #IMPLIED style %StyleSheet; #IMPLIED title %Text; #IMPLIED" >
Notice that entity %coreattrs further references entities (%StyleSheet; and %Text;) to define values for the style and title attributes. Once defined, the core attributes could be added to an attribute list declaration for an element, as follows:
<!ATTLIST some-element %coreattrs;>
Oftentimes, you will see entities that in turn contain further entities. For example, under HTML 4.0 the coreattrs parameter entity is used with the %i18n and events parameter entities to define the expansion text for an aggregate entity called attrs.
<!ENTITY % attrs "%coreattrs %i18n %events">