The same Greek word is translated by both the words expiation and propitiation from time to time. But there is a slight difference in the terms. Expiation is the act that results in the change of God’s disposition toward us. It is what Christ did on the cross, and the result of Christ’s work of expiation is propitiation—God’s anger is turned away. The distinction is the same as that between the ransom that is paid and the attitude of the one who receives the ransom. - R.C. Sproul
The difference is crucial at certain points in Scripture. Be sure you know the difference as not all theologians and not all Bible translations have it right. As Sproul notes...
Together, expiation and propitiation constitute an act of placation. Christ did His work on the cross to placate the wrath of God. This idea of placating the wrath of God has done little to placate the wrath of modern theologians. In fact, they become very wrathful about the whole idea of placating God’s wrath. They think it is beneath the dignity of God to have to be placated, that we should have to do something to soothe Him or appease Him. We need to be very careful in how we understand the wrath of God, but let me remind you that the concept of placating the wrath of God has to do here not with a peripheral, tangential point of theology, but with the essence of salvation.
Read the whole article from Ligonier Ministries here.