Expiation is the cancellation of sin. Expiation and propitiation are similar but expiation does not carry the implication of dealing with wrath, of appeasing it through a sacrifice. Generally speaking, propitiation cancels sin and deals with God's wrath. Expiation is simply the cancellation of sin. Jesus was our propitiation (1 John 2:2; 4:10 -- "atoning sacrifice" in the NIV).
Websters 1828 dictionary, "expiation, "the acto of atoning for a crime; the act of making satisfaction for an offense, by which the guilt is done away, and the obligation of the offended person to punish the crime is caneled."