What is expiation in general?

Expiation (Kaffarah): It is what the Shari'ah (Islamic law) prescribes as a rectification for a deficiency or an error, or as a deterrent from a sin and a transgression. It is what an offender seeks forgiveness through, such as charity, fasting, and the like. It is also applied to that which covers a sin. Its types are many, including expiation for breaking an oath, and expiation for violating the sanctity of a Ramadan day.