In a series of works, an installment that is set chronologically before its predecessor, especially the original narrative or (perhaps improper usage) any narrative work with at least one sequel
A prequel is a film that is made about an earlier stage of a story or a character's life when the later part of it has already been made into a successful film. `Fire Walk With Me', David Lynch's prequel to the TV series `Twin Peaks'. a book, film etc that tells you what happened before the story told in a previous popular book or film sequel prequel to (pre- + -quel (as in sequel))