the gold cover over the ark: the slab of pure gold on top of the chest in the innermost room, where the LORD met Israel and where blood was applied to make things right
The word names one object, and only one: the lid of solid gold that lay on top of the ark (אֲרוֹן) in the most holy place, with two cherubim hammered out of the same piece, their wings spread over it and their faces toward each other. Every one of its appearances is about this single thing, so it is not a word with several meanings. It is a word with one referent and a deep argument about what that referent was.
The argument is built into the word itself. It is formed from the same three letters as the verb "to make atonement" (or, on an older view, "to cover"), so the name can be heard two ways. It can name the object plainly, the cover that closes the chest holding the covenant tablets. Or it can name the object by what happens there, the place where atonement is made. The Bible uses it for both at once: it is the lid of a box, and it is the exact spot where God's voice came out from between the cherubim and where, once a year, blood was sprinkled to cleanse the sanctuary.
This is why a clean English word has never been found.
- "Cover" catches the object but loses the atonement.
- "Mercy seat" adds a throne and a tenderness the Hebrew does not say.
- "Place of atonement" catches the function but turns a thing into an event.
The senses below place all the occurrences under this one object, because the text never uses the word for anything else. Where scholars genuinely divide is recorded in the disputes: not over which verse means what, but over what the name was reaching for in the first place.