Mercy-seat : (Heb. kapporeth, a "covering;" LXX. and N.T., hilasterion; Vulg., propitiatorium), the covering or lid of the ark of the covenant (which see). It was of acacia wood, overlaid with gold, or perhaps rather a plate of solid gold, Matthew 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).