In Vampire: the Masquerade, any truly ancient (4000+ years) vampire, and specifically one of the thirteen or so founders of the clans. Note that House and Clan Tremere don't have a proper antediluvian, since their founder was an hermetic mage who decided to go for power the quick way, and committed diablerie on the head of another clan. A similar tale applies to clan Giovanni. Clan Assamite claim not to be descended from Caine, but their founder is still an antediluvian.