Prequel
For correct use, the term 'prequel' is only applied when work is part of a series, and when the later publication refers to an earlier time. It is an old idea, but one which is often used today for drama on movie and television. The Cypria, a work of Ancient Greek literature, is a prequel to Homer's Iliad. A related idea is the back-story, which is the background story of a main story or character.