In the Bible "atonement" refers to a ritual or act that covers sin and restores relationship with God, described in the sacrificial system of Leviticus.
Leviticus 17:11 in the King James Version reads:
For the life of the flesh is in the blood: and I have given it to you upon the altar to make an atonement for your souls: for it is the blood that maketh an atonement for the soul.
The verse appears in Leviticus 17, in the laws governing the eating of blood and the use of blood in sacrifice. Read it in context at Leviticus 17:11, or the full chapter at Leviticus 17. Encyclopedia entry: en.wikipedia.org/wiki/Atonement_in_Christianity.