Primer Pocket
The recess in the base of a brass case that holds the primer. Its dimensions and depth affect primer seating, ignition, and brass life.
A new primer pocket has a tight, square fit. Each firing very slightly expands the pocket; after 5-10 firings on hot loads, the pocket can become loose enough that primers seat with no friction, a sign the brass is at end-of-life for that load.
Handloaders sometimes uniform the primer pockets (cut them to a precise depth and square) and clean carbon out between firings. Small precision gains; meaningful on long-range match ammunition.