Breech
The rear end of the barrel where the cartridge is loaded and sealed, opposite the muzzle. The bolt or action closes against it.
The breech is the back of the barrel, where the chamber is cut and the cartridge sits ready to fire. It is the loading end and the sealing end: when the bolt or breechblock closes against it, the action contains the pressure of firing so all of it goes toward driving the bullet out the muzzle.
The fit at the breech is where headspace is set and where a rifle is either tight and accurate or loose and inconsistent. Loading from the breech is what separates modern rifles from the muzzleloaders they replaced.