Hades (from Greek , Hadēs, originally , Haidēs or , Aidēs, probably from Proto-Indo-European * 'unseen') refers both to the ancient Greek underworld, the abode of Hades, and to the god of the underworld. Hades in Homer referred just to the god; the genitive , Haidou, was an elision to denote locality: "house/dominion of Hades". Eventually, the nominative, too, came to designate the abode of the dead.

In Greek mythology, Hades and his brothers Zeus and Poseidon defeated the Titans and claimed rulership over the universe ruling the underworld, sky, and sea, respectively; the land was given to all three concurrently. Because of his association with the underworld, Hades is often interpreted as a grim figure.

Hades was also called Pluto (from Greek Ploutōn, from πλοῦτος, wealth), meaning "Rich One".

In Christian theology, the term hades refers to the abode of the dead or Sheol (also Hell), where the dead await Judgment Day either at peace or in torment (see Hades in Christianity below).

Realm of Hades

main: Greek underworld

There were several sections of the realm of Hades, including the Elysian Fields (contrast the Christian Paradise or Heaven), and Tartarus, (compare the Christian Hell). Greek mythographers were not perfectly consistent about the geography of the afterlife. A contrasting myth of the afterlife concerns the Garden of the Hesperides, often identified with the Isles of the Blessed, where the blessed heroes may dwell.

In Roman mythology, the entrance to the underworld located at Avernus, a crater near Cumae, was the route Aeneas used to descend to the Underworld. By synecdoche, "Avernus" could be substituted for the underworld as a whole. The Inferi Dii were the Roman gods of the underworld.

The deceased entered the underworld by crossing the Acheron, ferried across by Charon (kair'-on), who charged an obolus, a small coin for passage placed in the mouth of the deceased by pious relatives. Paupers and the friendless gathered for a hundred yearsFact: date=March 2009 on the near shore. Greeks offered propitiatory libations to prevent the deceased from returning to the upper world to "haunt" those who had not given them a proper burial. The far side of the river was guarded by Cerberus, the three-headed dog defeated by Heracles (Roman Hercules). Passing beyond Cerberus, the shades of the departed entered the land of the dead to be judged.

The five rivers of the Realm of Hades, and their symbolic meanings, are Acheron (the river of sorrow), Cocytus (lamentation), Phlegethon (fire), Lethe (forgetfulness), and Styx (hate). See also Eridanos. Styx forms the boundary between the upper and lower worlds.