Tartarus

From BibleWiki

Greek word occurring only in 2 Pet 2:4, although it is common in the classical writers as the lower part, or abyss of hades, where the shades of the wicked were supposed to be imprisoned and tormented. In Peter it is used as the destination of the angels that sin.

Most translate this simply as 'hell', but some (including the Jehovah's Witnesses) believe this to mean the lowest part of the abyss. It is often thought of as equivalent to Gehenna. It was regarded, commonly, as beneath the earth; as entered through the grave; as dark, dismal, gloomy; and as a place of punishment. Compare Job 10:21f, and see Mt 5:22.