|Easton's Bible Dictionary|
(Hebrews kapporeth, a "covering;" LXX. and N.T., hilasterion; Vulg., propitiatorium), the covering or lid of the ark of the covenant (q.v.). It was of acacia wood, overlaid with gold, or perhaps rather a plate of solid gold, 2 1/2 cubits long and 1 1/2 broad (Exodus 25:17; 30:6; 31:7). It is compared to the throne of grace (Hebrews 9:5; Ephesians 2:6). The holy of holies is called the "place of the mercy-seat" (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; Comp. Numbers 7:89 and Exodus 25:22).
Int. Standard Bible Encyclopedia
mur'-si-set (kapporeth; New Testament hilasterion, Hebrews 9:5): The name for the lid or covering of the ark of the covenant (Exodus 25:17, etc.). The Old Testament term means "covering," then, like the New Testament word, "propitiatory" (compare kipper, "to cover guilt," "to make atonement"). The ark contained the two tables of stone which witnessed against the sin of the people. The blood of sacrifice, sprinkled on the mercy-seat on the great day of atonement, intercepted, as it were, this condemning testimony, and effected reconciliation between God and His people. See ATONEMENT; ATONEMENT, DAY OF; PROPITIATION; ARK OF THE COVENANT. In Romans 3:25, Jesus is said to be set forth as "a propitiation (literally, "propitiatory"), through faith, in his blood," thus fulfilling the idea of the mercy-seat (compare Hebrews 9:5, 7, 11, 12, etc.).
W. Shaw Caldecott
Mercyseat (1 Occurrence)
Hebrews 9:5 And over it the cherubims of glory shadowing the mercyseat; of which we cannot now speak particularly. (KJV)