I found this to be helpful

I was struggling a bit with the difference between propitiation and expiation, so I looked it up in theopedia and this is what it says...

The Greek word hilasterion is the Greek rendering of the Hebrew kapporeth which refers to the Mercy Seat of the Ark. Hilasterion can be translated as either propitiation or expiation which then imply different functions of the Mercy Seat. Propitiation literally means to make favorable and specifically includes the idea of dealing with God’s wrath against sinners. Expiation literally means to make pious and implies either the removal or cleansing of sin.

The idea of propitiation includes that of expiation as its means; but the word "expiation" has no reference to quenching God’s righteous anger. The difference is that the object of expiation is sin, not God. One propitiates a person, and one expiates a problem. Christ's death was therefore both an expiation and a propitiation. By expiating (removing the problem of) sin God was made propitious (favorable) to us.

That makes sense to me, unless some Greek scholar types say otherwise...