A literary, dramatic, or cinematic work whose narrative takes place before that of a preexisting work.
A novel (or film, etc.) about events that preceded and, often, led up to those of another that was published (or produced) earlier.
(narratology) 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.
Origin of prequel
Patterned after sequel using pre- (“before").