Sine qua non
Latin for "without which not" - something that has to exist to bring other things into being. The third of Mark Kermode's fondly used Latinisms, the others being ne plus ultra and reducto ad absurdum.
Sin qua non is often used in the context of movies that rip off other movies, or cash in on their success. Lord Of The Rings is the sine qua non for Eragon; St Trinian's the sine qua non for Wild Child.