Deburring
Removing the sharp burr left on the outside of a case mouth after trimming. Usually paired with chamfering the inside edge.
Cutting a case to length leaves a raised burr on the outer rim of the mouth. Deburring shaves that ridge flush so the mouth is smooth, which helps the round chamber cleanly and keeps the burr from catching during feeding.
The outside deburr and the inside chamfer are usually done in one pass with a two-ended tool right after case trimming. Handloaders chasing consistency also deburr the flash hole inside the case, removing the punched ridge so the primer flame reaches the powder the same way every time.