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.

see GREEK hilaskomai

1. the well-known cover of the ark of the covenant in the Holy of holies, which was sprinkled with the blood of the expiatory victim on the annual day of atonement (this rite signifying that the life of the people, the loss of which they had merited by their sins, was offered to God in the blood as the life of the victim, and that God by this ceremony was appeased and their sins were expiated); hence, the lid of expiation, the propitiatory, Vulg.propitiatorium; Luth.Gnadensruhl (A. V. mercy-seat): Hebrews 9:5 (the Sept. Exodus 25:18ff; Leviticus 16:2, etc.; more fully ἱλαστήριον ἐπίθεμα, Exodus 25:17; Exodus 38:7 (Exodus 37:6), for the Hebrew כַּפֹּרֶת, from כִּפֶּר to cover, namely, sins, i. e. to pardon). Theodoret, Theophylact, Oecumenius, Luther, Grotius, Tholuck, Wilke, Philippi, Umbreit (Cremer (4te Aufl.)) and others give this meaning to the word also in Romans 3:25, viz. that Christ, besprinkled with his own blood, was truly that which the cover or 'mercy-seat' had been typically, i. e., the sign and pledge of expiation; but in opposed to this interpretation see Fritzsche, Meyer, Van Hengel (Godet, Oltramare) and others at the passage

2. an expiatory sacrifice; a piacular victim (Vulg.propitiatio): Romans 3:25 (after the analogy of the words χαριστηρια sacrifices expressive of gratitude, thank-offerings, σωτηρία sacrifices for safety obtained. On the other hand, in Dio Chrysostom or. 11, 121, p. 355, Reiske edition, the reference is not to a sacrifice but to a monument, as the preceding words show: καταλείψειν γάρ αὐτούς ἀνάθημα κάλλιστον καί μέγιστον τῇ Ἀθηνα καί ἐπιγράψειν, ἱλαστήριον Ἀχαιοι τῇ Ἰλιαδι). (See the full discussion of the word in Dr. Jets. Morison, Critical Exposition of the Third Chapter of the Epistle to the Romans, pp. 281-303.)