The OT's term for the realm of the dead — the underworld where all the deceased go, awaiting the resurrection. In the early OT, Sheol is often described as a shadowy place of minimal existence, cut off from praise of God. Later Psalms begin to express confidence that God's presence even reaches there and that God will not abandon the righteous to Sheol's power.
Psalm 16's climactic claim — 'You will not abandon my soul to Sheol' — is quoted in Acts 2 as a prophecy of Christ's resurrection. The NT uses Hades (Greek equivalent) similarly. While Sheol is sometimes used synonymously with death or the grave, the prophets begin to hint at resurrection: 'Your dead shall live' (Isaiah 26:19). The NT completes this: Christ holds the keys of Death and Hades (Rev 1:18).