Expiation

Guilt is said to be expiated when it is visited with punishment
falling on a substitute. Expiation is made for our sins when
they are punished not in ourselves but in another who consents
to stand in our room. It is that by which reconciliation is
effected. Sin is thus said to be "covered" by vicarious
satisfaction.

The cover or lid of the ark is termed in the LXX. hilasterion,
that which covered or shut out the claims and demands of the law
against the sins of God's people, whereby he became "propitious"
to them.

The idea of vicarious expiation runs through the whole Old
Testament system of sacrifices. (See PROPITIATION T0003007.)