American here, I've heard both "Map Key" (or just key) and "Legend" used interchangeably for map Keys/Legends for as long as I can recall. So I suppose there's

American here, I've heard both "Map Key" (or just key) and "Legend" used interchangeably for map Keys/Legends for as long as I can recall. So I suppose there's more to it than just American vs. British English.