(Heb. 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 (Ex. 25:17; 30:6; 31:7). It is compared to the throne
of grace (Heb. 9:5; Eph. 2:6). The holy of holies is called the
"place of the mercy-seat" (1 Chr. 28:11: Lev. 16:2).

It has been conjectured that the censer (thumiaterion, meaning
"anything having regard to or employed in the burning of
incense") mentioned in Heb. 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 (Lev. 16:11-16; compare Num. 7:89 and Ex. 25:22).