In Greek mythology Erebus (Έρεβος Erebos, "Deep blackness/darkness or shadow" from Ancient Greek Ἔρεβος) was the son of a primordial God, Chaos, the personification of darkness and shadow, which filled in all the corners and crannies of the world. He was the offspring of Chaos alone. He was brother of Nyx and father with her of Aether and Hemera, according to Hesiod (c. 700 BC). According to Hyginus (c. AD 1), he was the father of Geras.

According to some later legends, Erebus was part of Hades, the underworld, to the extent that the River Styx is referenced by Hesiod as the "Blood of Erebus." It was where the dead had to pass immediately after dying. After Charon ferried them across the river Acheron, they entered Tartarus, the underworld proper. Erebus was often used as a synonym for Hades, the Greek god of the underworld.

The word is probably cognate to Old Norse rœkkr, Gothic riqis "darkness", Sanskrit rajani "night", Tocharian orkäm "darkness". Another suggestion is a loan from Semitic, c.f. Hebrew erebh and Akkadian erebu "sunset, evening" (hence, "darkness"). The same etymology of "sunset" has been suggested for Europe.