The Greek verb hilaskomai (from hilaos, propitious/gracious) means to make propitiation, to appease divine wrath against sin, or to perform the act of atonement. It appears in Luke 18:13 ('God, be merciful [hilastheti] to me, a sinner!') and Hebrews 2:17 ('that he might make atonement [hilaskesthai] for the sins of the people'). The word is the Septuagint translation of kaphar (H3722, to cover/atone).
Hilaskomai sits at the very center of biblical soteriology. The tax collector's prayer in Luke 18:13 — 'God, hilastheti me, the sinner' — is the cry of one who knows he cannot atone for himself; he throws himself on God's mercy. Hebrews 2:17 reveals the answer: Jesus became fully human specifically 'that he might make atonement for the sins of the people.' He is both the High Priest who performs the act (hilaskesthai) and the Lamb who is the offering. The wrath of God against sin is not dismissed but fully and finally absorbed in Christ.