"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.

In an extended flashback, after the death of his wife, Richard Alpert (Nestor Carbonell) arrives on the island as a slave on the Black Rock in the 19th century. He then encounters the Man in Black (Titus Welliver) and Jacob (Mark Pellegrino), both of whom offer him different deals.

The title is Latin for "from eternity". The phrase is used to mean "since the beginning" or "for long ages". "Ab Aeterno" was watched by 9 million American viewers and received critical acclaim by critics and audiences alike.