A propitiatory: a covering for the ark: called a propitiatory, or mercy seat, because the Lord, who was supposed to sit there upon the wings of the cherubims, with the ark for his footstool, from thence showed mercy. It is also called the oracle, ver. 18 and 20; because from thence God gave his orders and his answers.
[AD 1781] Richard Challoner on Exodus 25:17