Sōtērion (G4992) is the neuter form of the adjective sōtērios (bringing salvation) used as a noun: 'the salvation' or 'the saving thing.' It emphasizes salvation as a concrete, objective reality — the accomplished deliverance wrought by God.
Simeon's famous prayer uses this word: 'My eyes have seen your salvation (sōtērion)' (Luke 2:30) — he holds the incarnate Savior and declares the long-awaited deliverance has arrived. Paul quotes Isaiah 52:10 in Acts 28:28: 'God's salvation (sōtērion) has been sent to the Gentiles.' The word emphasizes not merely the concept but the concrete event and person of salvation: Jesus Himself is the sōtērion.