Real Dictionary

prequel
countable noun: A prequel is a film, book, or television series 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, book, or series.

prequel in American English
a film, novel, etc. about events that preceded and, often, led up to those of another novel, film, etc. that was produced or published earlier
noun: a literary, dramatic, or filmic work that prefigures a later work, as by portraying the same characters at a younger age

prequel in British English
noun: a film, novel, or play that portrays events that occurred before the events of an existing work