Hastings' Dictionary of the New Testament 
(ἱλαστήριον, propitiatorium )
The mercy-seat was the cover of the Ark ( q.v. [Note: .v. quod vide, which see.]) of the Covenant in the Holy of Holies. It was sprinkled with the blood of the victim slain on the annual Day of Atonement ( Hebrews 9:5). ‘Mercy-seat’ is admitted on all hands to be an imperfect translation of the Greek word, being rather, like Luther’s Gnadenstuhl , equivalent to θρόνος τῆς χάριτος ( Hebrews 4:16). It is also frequently contended that ἱλαστήριον, which is the Septuagintrendering of כַּפֹּרָת, is itself a mistake. In the view of Rashi and Kimchi, followed by many Christian scholars, the Heb. word means no more than a literal ‘covering’ (so Revised Version margin in Exodus 25:17, etc.). Ritschl maintains that in both the OT and the NT ἱλαστήριον designates ‘the piece of furniture over the ark of the covenant in the holy of holies’ ( Rechtfertigung und Versöhnung 3, ii.  168). Nowack ( Heb. Archäologie , 1894, ii. 60 f.) also gives the word a material sense, regarding it, however, as denoting a kind of penthouse ( Schutzdach, Deckplatte ) for the ark. But the analogy of the Arabic kaffârat seems to justify Lagarde (and many others) in holding (1) that the Septuaginthas rendered the original quite accurately, and (2) that ἱλαστήριον means ‘the propitiating thing,’ or ‘the propitiatory gift.’ Wherever the word is used by Philo ( de Vit. Mos. iii. 8, de Profug. 19, de Cherub. 8, etc.) this is the meaning indicated by the context, and recently discovered inscriptions (W. R. Paton and E. L. Hicks, The Inscriptions of Cos , 1891) prove that ἱλαστήριον ordinarily bore this sense in the early Imperial period (cf. Dio Chrysostom, Or. xi. 355 [Reiske]).
With such a connotation the word lies at the heart of St. Paul’s gospel ( Romans 3:25). When he depicts Christ Jesus as set forth to be a ἱλαστήριον (or his word may be an adj., ἱλαστήριος), it is scarcely possible that he conceives the Messiah as a ‘mercy-seat,’ or ‘covering of the ark,’ sprinkled with blood-His own blood. The figure is inappropriate and unintelligible. But the Apostle’s thought is at once apparent and impressive if he represents Christ as a Propitiatory. The exact shade of meaning which may thereafter be detected in the word-whether ‘the means of propitiating,’ or ‘the propitiatory gift,’ or ‘the propitiatory One’-is of less importance. What is essential is the large and luminous idea of atonement. The Pauline teaching and the Johannine are here in agreement, each emphasizing the same central thought. Christ as the ἱλαστήριον (propitiatory) is the ἱλασμός (propitiation) for our sins ( 1 John 2:2).
Literature.-P. de Lagarde, Uebersicht über die im Aram., Arab. und Heb. übliche Bildung der Nomina , Göttingen, 1889; H. Cremer, Bibl.-theol. Wörterbuch 8, Gotha, 1895, p. 474 ff.; G. A. Deissmann, Bible Studies , Eng. translation, 1901, p. 124 ff., also articlein Encyclopaedia Biblica .
Hawker's Poor Man's Concordance And Dictionary 
Much is spoken of in the Old Testament Scripture concerning this sacred part of the temple, from whence the Lord promised to commune with his people. ( Exodus 25:17, etc.) This, as a type of the Lord Jesus, is eminently to be regarded, since it serves to teach us, that by efficacy of redemption, the Old Testament saints, as well as New Testament believers, were alike included in the merits of "the Lamb slain from the foundation of the world." ( Revelation 13:8)
The form of the mercy-seat, or propitiatory, was that of an ark, covered with gold, at the two ends of which were placed the cherubim to cover over the mercy-seat, from whence Jehovah was supposed to speak. ( Psalms 80:1) The apostle Paul gives a short description of the tabernacle, and the furniture in it, ( Hebrews 9:1) etc.â€”and speaking of the cherubim of glory shadowing the mercy seat, he saith, "of which we cannot now speak particularly." The Hebrews called the mercy-seat Caphoreth, from the word Caphar, to expiate or pardon. And very probably the church had this in view when she said: "My beloved is unto me as a cluster of camphire (copher) in the vineyards of Engedi." ( Song of Song of Solomon 1:14) If, as it is believed, that it is Christ she is then praising, with an eye to his propitiation, when she thus expressed herself, it is very striking and beautiful. Jesus is indeed the true and only propitiatory and propitiation; and what a sweet addition to the blessed subject is it, that he is "the propitiation whom God the Father hath set forth through faith in his blood!" So that our faith finds a double warrantâ€”first, in the completeness of the propitiation itself, and, secondly, in God's appointment of it. And how can a soul come short of salvation that acts faith upon the infinite merits of God the Son's righteousness, and the infinite faithfulness of God the Father's grace?
People's Dictionary of the Bible 
Mercy-seat was the name of the lid or cover of the ark of the covenant. It was made of gold, two and a half cubits long and one and a half cubits broad, and two cherubs, also of gold, were placed one at each end, stretching their wings toward each other, and forming a kind of throne, upon which God was believed to be present in a peculiar manner to hear and answer prayer, and to make known his holy will. Exodus 25:17-22; Exodus 30:6; Exodus 31:7; Exodus 37:6-9; 1 Chronicles 28:11; 2 Chronicles 5:7-8; Psalms 80:1; Psalms 99:1. Before and upon the mercy-seat the high priest sprinkled the blood of the sin-offerings on the day of atonement as a propitiation, Leviticus 16:11-16, which, under the new dispensation, received its fulfillment. Hebrews 9:5; Romans 3:25.
Easton's Bible Dictionary 
Exodus 25:17 30:6 31:7 Hebrews 9:5 Ephesians 2:6 1 Chronicles 28:11 Leviticus 16:2
It has been conjectured that the censer (thumiaterion, meaning "anything having regard to or employed in the burning of incense") mentioned in Hebrews 9:4 was the "mercy-seat," at which the incense was burned by the high priest on the great day of atonement, and upon or toward which the blood of the goat was sprinkled ( Leviticus 16:11-16; Compare Numbers 7:89 and Exodus 25:22 ).
American Tract Society Bible Dictionary 
1 Chronicles 28:11 , the cover of the Ark of the Covenant, which see. The Hebrew word means a cover, but contains an allusion to the covering or forgiving of sins, Psalm 32:1 . In the New Testament it is designated by a Greek word meaning "the propitiatory," or "expiatory," Hebrews 9:4,5 . It was approached only by the high priest, and not without the blood of atonement, to show that the divine mercy can be granted only through the blood of Christ, Romans 3:25 .
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
( כִּפֹּרֶת , Kappo'Reth , a Covering , i.e. lid of a vessel, spoken only of the top of the sacred ark; ‘ Sept. and, New Test. Ἱλαστήριον , Vulg, Propitiatorium ), the cover of the box or ark containing the tables of the Sinaitic law, and overspread by the cherubim, between which appeared the shekinah, or visible radiant symbol of the divine presenite; it is properly represented as a plank of acacia overlaid with gold, for it was not probably a solid plate or sheet of the purest gold ( Exodus 25:17 sq.; Exodus 30:6; Exodus 31:7, etc.). Hence the holy of holies is sometimes called the "house of the mercy- seat" ( 1 Chronicles 28:11, Heb.). Josephus simply calls it a lid ( Ἐπίθεμα , Ant . 3:6, 5); but the versions have all regarded the term as indicative of Propitiation (as if from the Piel of כָּפִר , and the same view appears to be taken by the New-Testament writers, who compare it with the throne of grace in heaven, access to which has been opened by the blood of Christ ( Hebrews 9:5; Romans 3:24). (See Ark). Comp. 1 Chronicles 28:11, where the holy of holies is called the הִכִּפֹּרֶת בֵּית , "house of the mercy-seat." "It was that whereon the blood of the yearly atonement was sprinkled by the high-priest; and in this relation it is doubtful whether the sense of the word in the Hebrews is based on the material fact of its ‘ covering' the ark, or from this notion of its reference to the ‘ covering' (i.e. atonement of sin. (See Atonement) . But in any case the notion of a ‘ seat,' as conveyed by the name in English, seems superfluous and likely to mislead. Jehovah is indeed spoken of as ‘ dwelling' and even as ‘ sitting' ( Psalms 80:1; Psalms 99:1) between the cherubim, but undoubtedly his seat in this conception would not be on the same level as that on which they stood ( Exodus 25:18), and an enthronement in the glory above it must be supposed. The idea with which it is connected is not merely that of ‘ mercy,' but of formal atonement made for the breach of the covenant ( Leviticus 16:14), which the ark contained in its material vehicle-the two tables of stone. The communications made to Moses are represented as made ‘ from the mercy- seat that was upon the ark of the testimony' ( Numbers 7:89; comp. Exodus 25:22; Exodus 30:6); a sublime illustration of the moral relation and responsibility into which the people were by covenant regarded as brought before God" (Smith). It is not without significance that the mercy-seat was Above the ark and below the symbols of the divine presence and attributes, as if to foreshadow the supersedence of the law of ordinances contained in the ark by the free grace of the Gospel. See Pratenius, De Judcea Arca (Upsal. -1727); Werner, De Propitiatoria (Giessen, 1695). (See Shekinah).
Kitto's Popular Cyclopedia of Biblial Literature 
The Hebrew name literally denotes a cover, and, in fact, describes the lid of the ark with cherubim, over which appeared 'the glory of God' (, sq.; 37:8; 31:7, and elsewhere) [ARK]. The word used in the Septuagint and New Testament to translate this term, signifies the 'expiatory' or 'propitiatory,' in allusion to that application of the Hebrew word which we have noted: which application is in this instance justified and explained by reference to the custom of the high-priest once a year entering the most holy place, and sprinkling the lid of the ark with the blood of an expiatory victim, whereby 'he made atonement for the sins of the people.' As this was the most solemn and significant act of the Hebrew ritual, it is natural that a reference to it should be involved in the name which the covering of the ark acquired. By a comparison of the texts in which the word occurs, it will be seen that there would, in fact, have been little occasion to name the cover of the ark separately from the ark itself, but for this important ceremonial.
- Mercy-Seat from Hastings' Dictionary of the New Testament
- Mercy-Seat from Hawker's Poor Man's Concordance And Dictionary
- Mercy-Seat from People's Dictionary of the Bible
- Mercy-Seat from Easton's Bible Dictionary
- Mercy-Seat from American Tract Society Bible Dictionary
- Mercy-Seat from Cyclopedia of Biblical, Theological and Ecclesiastical Literature
- Mercy-Seat from Kitto's Popular Cyclopedia of Biblial Literature