The satisfaction of a legitimate demand. In a more restricted sense it is the reparation of an offense. This occurs through a voluntary performance that outweighs the injustice done. If the performance fully counterbalances the gravity of the guilt, the atonement is adequate. And if the atonement is done by someone other than the actual offender, but in his stead, it is vicarious.
Applied to Christ the Redeemer, through his suffering and death he rendered vicarious atonement to God for the sins of the whole human race. His atonement is fully adequate because it was performed by a divine person. In fact, it is superabundant because the positive value of Christ's expiation is actually greater than the negative value of human sin. (Etym. Middle English at one, to set at one, to reconcile; of one mind, in accord.)