Mercy Seat

... Christ Jesus, whom God set forth as a propitiation by His blood ...  (Romans 3)

So, that "propitiation" is another word for "mercy seat."

In the Holy of Holies, behind the veil in the tabernacle and the temple, the ark of the covenant was covered by the mercy seat.  The place of the Lord's presence.  The place where He particularly was with His people to forgive.  Tucked away.  Nobody went there.  Except the high priest once a year.  With blood and with cleansing.

And now, in the fulness of time, God set forth the propitiation.  Not tucked away.  Right out there where the whole world can see.