The LXX rendered the kappōreth (the golden lid of the ark where the Day-of-Atonement blood was sprinkled, Lev 16:14–15) as hilastērion. The word then surfaces exactly twice in the New Testament: Hebrews 9:5, describing the literal furniture ("the cherubims of glory shadowing the mercyseat," KJV), and Romans 3:25, where God sets forth Christ "to be a propitiation through faith in his blood" (KJV).
"Jesus Christ is the great propitiation, or propitiatory sacrifice, typified by the hilasterion, or mercy-seat, under the law." — Matthew Henry, Commentary on the Whole Bible (1710), on Romans 3:25–26.
Whether the word-group means propitiation (wrath turned away: Morris, 1955) or expiation (contamination purged: Dodd; Milgrom) is a live lexical dispute, steelmanned in full on the sacrificial-system page's contested drawer.
This site's tradition stands with Morris here: penal substitution is the controlling atonement category, so hilastērion at Romans 3:25 is read as propitiation, wrath righteously borne and turned away, not merely contamination purged, while the expiation reading is held honestly rather than dismissed. The workbench: hilastērion, G2435. Sources: KJV (PD); Matthew Henry, Commentary on the Whole Bible (1710); MannaFest's research index.