From kaphar; a lid (used only of the cover of the sacred Ark) -- mercy seat.
see HEBREW kaphar
H3727. kapporeth
כַּמֹּרֶת noun propitiatory, late technical word from כּפר cover over sin: the older explanation cover, lid has no justification in usage; ᵐ5 ιλαστήριον; only P: Exodus 25:17-18, 19, 20 (twice in verse); 25:21-22, 26:34; 30:6; 31:7; 35:12; 37:6-7, 8, 9 (twice in verse); 39:35; 40:20; Leviticus 16:2 (twice in verse); 16:13-14, (twice in verse); 16:15 (twice in verse); Numbers 7:89, and 1 Chronicles 28:11; it was a slab of gold 22-Jancubits x 1 2-Jancubits placed on top of the ark of the testimony. On it, and a part of it, were two golden cherubim facing each other, whose outstretched wings came together above and constituted the throne of Yahweh. When the high priest entered the Holy of Holies on the day of atonement it was necessary that this highest place of atonement should be enveloped in a cloud of incense. The blood of the sin-offering of the atonement was then sprinkled on the face of and seven times before it. The temple proper, as distinguished from porch etc., was called הַכּ ׳בֵּית 28:11.
II. כפר (√ of following).