PREQUEL
Etymology
Noun
prequel (plural prequels)
(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.
Usage notes
Most often used, not as a direct antonym of sequel, to refer to earlier works in a series, but to refer to works that are chronologically before but are created and released after; an archetypal example is the Star Wars prequel trilogy.
Antonyms
• sequel
Coordinate terms
• midquel
Source: Wiktionary