Atonement, n.
1. (Literally, a setting at one.) Reconciliation; restoration of friendly relations; agreement; concord. [Archaic] By whom we have now received the atonement. Rom. v. 11. He desires to make atonement Betwixt the Duke of Gloucester and your brothers. Shak.
2. Satisfaction or reparation made by giving an equivalent for an injury, or by doing of suffering that which will be received in satisfaction for an offense or injury; expiation; amends; -- with for. Specifically, in theology: The expiation of sin made by the obedience, personal suffering, and death of Christ. When a man has been guilty of any vice, the best atonement be can make for it is, to warn others. Spectator. The Phocians behaved with, so much gallantry, that they were thought to have made a sufficient atonement for their former offense. Potter.
Shades of meaning
Fernald has no entry of its own for atonement; he lists it under propitiation.
Where he writes about it:
Atonement (at-one-ment), originally denoting reconciliation, or the bringing into agreement of those who have been estranged, is now chiefly used, as in theology, in the sense of some offering, sacrifice, or suffering sufficient to win forgiveness or make up for an offense; especially and distinctively of the sacrificial work of Christ in his humiliation, suffering and death. — from propitiation