Case Head
The thick base of the cartridge case that holds the primer pocket and headstamp and is supported by the bolt face during firing.
The case head is the solid base of the cartridge, the thickest brass in the whole case because it has to contain the full force of the burning charge. It carries the primer pocket at its center, the flash hole that vents ignition into the powder, and the headstamp markings that identify caliber and maker. The rim or extractor groove that the case head presents is what the extractor grabs to pull the empty out of the chamber.
When the round fires, the case head bears directly against the bolt face, which holds it in place while internal pressure tries to drive it rearward. A head that is properly supported keeps the case from flowing or rupturing, so inspecting the head for pressure signs like flattened primers or shiny ejector marks is a routine part of working up a safe load.