Crimp
Squeezing the case mouth inward against the seated bullet to lock it in place. Comes in roll-crimp and taper-crimp varieties.
Roll crimps fold the case mouth into a cannelure groove on the bullet. Taper crimps just close the case mouth uniformly without forming a hook.
Precision rifle handloads usually don’t crimp at all. The brass-bullet friction (case neck tension) holds the bullet steady. Crimping is more common in rifles with heavy recoil, tubular magazines, or in autoloaders where feed cycles can push the bullet deeper.