In a generic sense, atonement is the act of making amends or reparation for a wrong or injury; it is to reconcile.
When considering God, atonement then becomes the action by which we are made right with him for the wrong things we have done.
Jesus provided that atonement for us when he was executed for our crimes (that is, when he died for our sins).
Prior to Jesus, in the Old Testament, the priests offered periodic sacrifices as a temporary atonement for the sins of the people.
Key verse about Atonement: For seven days make atonement for the altar and consecrate it. Then the altar will be most holy, and whatever touches it will be holy (Exodus 29:37, NIV)
A lifelong student of the Bible, Peter DeHaan, PhD, wrote the 1,000-page website ABibleADay.com to encourage people to explore the Bible. His main blog and many books urge Christians to push past the status quo and reconsider how they practice their faith in every area of their lives.