You can use a single entity to represent a lengthy declaration and then use the entity in the DTD. You can also use entities to make one common file that contains a set of standard declarations that can be shared by many DTDs.
Entities are like macros in the C programming language in that they allow you to associate a string of characters with a name. This name can then be used in either the DTD or the XML document; the XML parser will replace the name with the string of characters. All entities consist of three parts: the word ENTITY, the name of the entity (called the literal entity value), and the replacement text-that is, the string of characters that the literal entity value will be replaced with. All entities are declared in either an internal or an external DTD.