Expiation means to remove something.
In biblical theology it has to do with taking away or removing guilt by means of paying a ransom or offering an atonement. It means to pay the penalty for something. Thus, the act of expiation removes the problem by paying for it in some way, in order to satisfy some demand. Christ's expiation of our sin means that He paid the penalty for it and removed it from consideration against us. On the other hand, propitiation has to do with the object of the expiation.