Strong’s Greek
G2435 — hilasterion
The entry below is Strong’s own definition (1890, public domain), and every verse where the word occurs.
ἱλαστήριον
hilasterion — pronounced hil-as-tay'-ree-on
neuter of a derivative of 2433; an expiatory (place or thing), i.e. (concretely) an atoning victim, or (specially) the lid of the Ark (in the Temple):--mercyseat, propitiation. see GREEK for 2433
2 occurrences in the KJV.
Romans 3:25
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;
Hebrews 9:5
And over it the cherubims of glory shadowing the mercyseat; of which we cannot now speak particularly.