Dictionary.com

1. satisfaction or reparation for a wrong or injury; amends.
2. (sometimes initial capital letter) Theology. the doctrine concerning the reconciliation of God and humankind, especially as accomplished through the life, suffering, and death of Christ.
3. Christian Science. the experience of humankind’s unity with God exemplified by Jesus Christ.
4. Archaic. reconciliation; agreement.

Etymonline.com

1510s, “condition of being at one (with others),” a sense now obsolete, from atone + -ment. Theological meaning “reconciliation” (of man with God through the life, passion, and death of Christ) is from 1520s; that of “satisfaction or reparation for wrong or injury, propitiation of an offended party” is from 1610s.

Discussion/Explanation

When we wrong someone, commit a crime, etc., we have to atone for it (sooner or later). The information above does a great job of describing atonement.

Atonement in Christianity is all the more powerful as the atonement, the one who paid the price to make amends for mankind’s sins, was Christ. He paid the penalty in order to mend the broken relationship between mankind and God. This is why it is only through Christ that we can have any hope. There is no one else we can turn to in order to be right with God.