prequel

noun

1. 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.

   “'Prequels' are sequels that relate the story that preceded the original film.”

   “2008, 26 February, Andrew Pierce, "JRR Tolkien's estate to sue Lord of the Rings filmmakers New Line Cinema over profits, in The Daily Telegraph They are also threatening to block the production of the long-awaited prequel, The Hobbit, which may now be cancelled.”