pre ‧ quel /ˈpriːkwəl/ BrE AmE noun [countable]
[ Date: 1900-2000 ; Origin: pre- + -quel (as in sequel ) ]
a book, film etc that tells you what happened before the story told in a previous popular book or film ⇨ sequel
prequel to
‘The Phantom Menace’ is a prequel to ‘Star Wars’.