Brown Driver Briggs

 1. mercy-seat, place of atonement
  1) the golden plate of propitiation on which the High Priest
sprinkled the seat 7 times on the Day of Atonement symbolically
reconciling Jehovah and His chosen people
1a1. the slab of gold on top of the ark of the covenant which
measured 2.5 by 1.5 cubits; on it and part of it were the
two golden cherubim facing each other whose outstretched
wings came together above and constituted the throne of God