A DTD entity is a constant value declared in a DTD schema that can be referenced anywhere.
Syntax
<!DOCTYPE printers [
<!ELEMENT printers (#PCDATA)>
<!ATTLIST printer ink CDATA>
<!ENTIY startingInk "40 units">
]>
<printers>
<printer ink=&startingInk;></printer>
<printer ink=&startingInk;></printer>
<printer ink="20 units"></printer>
</printers>