The most common definition for the word "atonement" is reparation or satisfaction for an injury or a wrong. The word is a noun and has a few additional definitions.
The second definition for "atonement" is based in theology as the doctrine the concerns the reconciliation between God and mankind. An example of this definition is the biblical story of Jesus atoning for mankind's sins through his life and death.
The archaic definition is reconciliation or an agreement.
The word has its origins in the 16th century and is a combination of medieval Latin and middle English. It comes from the phrase "at one," meaning harmony, and adds the suffix "ment."