Boxer Primer
Also: Berdan Primer
The single-flash-hole primer design with a self-contained anvil that makes brass easy to decap and reload, unlike the twin-vent Berdan type.
A Boxer primer is a primer that carries its own anvil inside the cup and feeds its spark through a single, central flash hole into the case. Because there is one centered hole, a decapping pin can push straight through and pop the spent primer out the bottom, which is what makes Boxer-primed brass so straightforward to reload. It is the dominant design in American ammunition and the reason handloading is routine there.
The alternative is the Berdan primer, which has no internal anvil and instead uses an anvil formed in the case head with two or more offset flash holes feeding around it. Berdan brass can be reloaded, but it cannot be decapped with a simple center pin and the primer pocket sizes vary, so it sees far less reloading in practice. The two systems are not interchangeable, so it pays to check which one a given case uses before you try to deprime it.