Rim Break, Rim Diebreak. A die crack starting near the rim of a die and growing until a part of the die breaks away. It begins at the rim because this is where the metal stress is the greatest (between and around lettering and the rim) and usually proceeds on the surface toward the center. With continued striking it progresses into the body of the die. Two nearby diecracks may joint together and intensify until the die completely deteriorates. At the point when a small portion falls away – leaving a cavity in the die – the die is no longer useable. On the struck piece this die creates a raised lump or boss on the rim or between the legend and the rim. The resulting boss on the struck piece is called a cud by collectors. See broken die.

Reference:

NM19 {1991} Margolis, p 219-224.