Neuter of a derivative of hilaskomai; an expiatory (place or thing), i.e. (concretely) an atoning victim, or (specially) the lid of the Ark (in the Temple) -- mercyseat, propitiation.
KJV: Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;
NASB: whom God displayed publicly as a propitiation in His blood through faith. This was to demonstrate His righteousness, because in the forbearance of God He passed over the sins previously committed;