Exodus 25:17

And thou shalt make a mercy-seat, a lid, or covering, of the ark, of pure gold; two cubits and a half shall be the length thereof, and a cubit and a half the breadth thereof. This mercy seat, or lid of expiation, received its name from the fact that the guilt of the people, by virtue of the blood sprinkled against it on the great Day of Atonement, was expiated, their sin was covered.