"Ab Aeterno" is the ninth television episode of the American Broadcasting Company's sixth season of the serial drama television series Lost and 112th episode overall. The episode aired on March 23, 2010. The episode was written by producer Melinda Hsu Taylor and co-producer Greggory Nations and directed by Tucker Gates. The episode is centered on Richard Alpert and is the first episode in season six to return to the old traditional flashback and to not show a "flash-sideways". Despite Alpert being the centric character, centric flashbacks are also shown for Ilana Verdansky and The Man in Black (furthermore, Jacob plays large roles in all three characters' backstories).

Ab aeterno è una locuzione latina dal significato letterale di "Dall'eternità" e quindi "Da tempo immemorabile", usata specialmente in teologia in relazione con la generazione eterna del Verbo, e in filosofia.