Lexical Definition

‎ἱλαστήριον‎ hilastḗrion (hil-as-tay'-ree-on) propitiation ^G:N-N
neuter of a derivative of ‎ἱλάσκομαι‎

mercyseat, propitiation.

• an expiatory (place or thing), i.e. (concretely) an atoning victim, or (specially) the lid of the Ark (in the Temple)

Abbott-Smith

† ἱλαστήριος, -α, -ον (< ἱλάσκομαι), [in LXX: IV Mac 17:22; neut., Ex 25:16*(17*) ff. 31:7 35:12* 38:5* (37:6*) ff., Le 16:2, 13 ff. Nu 7:89 (כַּפֹּרֶת), Ez 43:14, 17, 20 (עֲזָרָה) Am 9:1 (on the original here, v. Deiss., BS, 127)*;]
propitiatory (μνῆμα, FlJ, Ant., xvi, 7, 1; θάνατος, II Mac, l.c.): of Christ, Ro 3:25; as subst., το ἱ. (sc. ἐπίθεμα, Ex 25:16*, 17*, where the word first occurs in LXX and where ἱ. ἐ. = כַּפֹּרֶת (q.v. in BDB, s.v.), elsewhere rendered simply τὸ ἱ; cf. Deiss., 124 ff.; Westc., He., in l.), He 9:5.†

Abbott-Smith References

Liddell-Scott-Jones

ἱλαστήριος-τήριος, α, ον

(ος, ον [Refs 2nd c.AD+], propitiatory, offered in propitiation, μνῆμα [LXX+1st c.AD+]; θυσίαι [Refs]

__II ἱλαστήριον ἐπίθεμα, the mercy-seat, covering of the ark in the Holy of Holies, [LXX] alone as substantive,[LXX+NT+1st c.AD+]
__II.2 (i.e. ἀνάθημα) propitiatory gift or offering, [NT]; of a monument, [Refs]
__II.3 monastery, Men.Prot.p.15 [Refs 4th c.BC+]

_{LSJ Scripture Refs: LXX.Exod.25.16* ☩ LXX.Lev.16.2 ☩ Heb.9.5 ☩ Rom.3.25}