Atonement is a cover or covering for sin (Hebrew kippur; French coiffure; English coffer, cover). The sin or blot can never be removed, it can only covered or atoned for.
In Christian and Hebrew doctrine atonement is the satisfaction or payment of a penalty. It can be reparation or making amends.
Atonement is often used to mean the reconciliation of God and man through the death of Jesus Christ. Christ died once for all (Heb. 6:10, 1 Pet. 3:18) to atone for sin. To reject this axiom is to leave a myriad of sins, both personally and collectively of a society, unaccounted for and exposed.
A Mormon view of the Atonement, as relayed by an LDS Elder: The Atonement of Jesus Christ, Elder Jeffrey R. Holland, jesuschrist.lds.org, Accessed March 2, 2008