The movable piece which closes the breech of a breech-loading firearm, and resists the backward force of the discharge. It is withdrawn for the insertion of a cartridge, and closed again before the gun is fired.

That part of the action which, being locked into position, supports the cartridge in the chamber of the gun, so that the case may form an effective gas seal when the weapon is discharged. Although a bolt is a breechblock it is not normally so called.

a metal block in breech-loading firearms that is withdrawn to insert a cartridge and replaced to close the breech before firing

The part of the action that fits securely against the cartridge keeping it snug in the breech and locks the action to allow maximum efficiency and safety during firing.

A breechblock (or "breech block") is the part of the firearm action that closes the breech of a weapon (whether small arms or artillery) at the moment of firing.