The rim of a crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

is the part that extends above the height of the local surface, usually in a circular or elliptical pattern. In a more specific sense, the rim may refer to the circular or elliptical edge that represents the uppermost tip of this raised portion. If there is no raised portion, the rim simply refers to the inside edge of the curve where the flat surface meets the curve of the crater bottom.