1. The mechanism of some firearms that holds the cartridge in place during the firing process. It must be moved out of the way to load and unload the gun; this action may be manually performed by the shooter pulling back on an exterior knob called the bolt handle and then sending it forward again, or the action may be performed by other moving parts within the firearm. When the user must move the bolt manually, the firearm is called a bolt-action firearm.

2. A cylindrical shaft, controlled by an attached lever, which rotates a partial revolution engaging locking lugs in complementary recesses, contains an internal spring-loaded firing pin, and becomes the breech-block of a bolt-action firearm.