<!entity entityname "string">
<!entity entityname FILE "filename">
An entity name can contain up to 64 letters, digits, and hyphens. Case is not significant in entity names, but is often used to improve readability for the author. The first character must be a letter. No space is permitted between the < ( left angle bracket), ! (exclamation mark), and entity in an <!entity> declaration.
Where you want the defined entity to appear, insert an entity reference using this syntax:
The entity reference consists of an & (ampersand), followed by the entity name (as defined in the entity declaration), and ends with a ; (semicolon).