Easton's 1897 Bible Dictionary -
"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 exercise 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 propitiation 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."
"My little children, I am writing these things to you so that you may not sin. But if anyone does sin, we have an advocate with the Father, Jesus Christ the righteous. He is the propitiation for our sins, and not for ours only but also for the sins of the whole world."
1 John 2:1-2