From Longman Dictionary of Contemporary Englishprequelpre‧quel /ˈpriːkwəl/ noun [countable] TCNABEFOREa book, film etc that tells you what happened before the story told in a previous popular book or film → sequelprequel to ‘The Phantom Menace’ is a prequel to ‘Star Wars’.
Examples from the Corpus
prequel• Michael Walsh is writing the Warner novel, which includes a prequel and a sequel to the events in the 1942 film.• "The Phantom Menace" is a prequel to "Star Wars."From Longman Business Dictionaryprequelpre‧quel /ˈpriːkwəl/ noun [countable] a book, film, play etc that tells an earlier part of the story in a book etc that already existsthe new prequel movies to ‘Star Wars’ → compare sequelOrigin prequel (1900-2000) pre- + -quel (as in sequel) 