Propitiation

that by which God is rendered propitious, i.e., by which it
becomes consistent with his character and government to pardon
and bless the sinner. The propitiation does not procure his love
or make him loving; it only renders it consistent for him to
execise his love towards sinners.

In Rom. 3:25 and Heb. 9:5 (A.V., "mercy-seat") the Greek word
"hilasterion" is used. It is the word employed by the LXX.
translators in Ex. 25:17 and elsewhere as the equivalent for the
Hebrew "kapporeth", which means "covering," and is used of the
lid of the ark of the covenant (Ex. 25:21; 30:6). This Greek
word (hilasterion) came to denote not only the mercy-seat or lid
of the ark, but also propitation or reconciliation by blood. On
the great day of atonement the high priest carried the blood of
the sacrifice he offered for all the people within the veil and
sprinkled with it the "mercy-seat," and so made propitiation.

In 1 John 2:2; 4:10, Christ is called the "propitiation for
our sins." Here a different Greek word is used (hilasmos).
Christ is "the propitiation," because by his becoming our
substitute and assuming our obligations he expiated our guilt,
covered it, by the vicarious punishment which he endured. (Compare
Heb. 2:17, where the expression "make reconciliation" of the
A.V. is more correctly in the R.V. "make propitiation.")