A lever action is a type of action for repeating firearms that uses a manually operated cocking handle located around the trigger guard area that pivots forward to move the bolt via internal linkages, which will feed and extract cartridges into and out of the chamber, and cock the firing pin mechanism. This contrasts to other type of repeating actions such as the bolt-action, pump-action, semi-automatic, fully automatic, and/or burst mode actions. A firearm using this operating mechanism is colloquially referred to as a levergun.
The toggle-link action used in the iconic Winchester Model 1873 rifle, one of the most famous lever-action firearms
Colt-Burgess rifle
Colt Paterson Ring Lever rifle
Inside of Marlin 39A receiver
In firearms terminology, an action is the functional mechanism of a breech-loading firearm that handles the ammunition cartridges, or the method by which that mechanism works. Actions are technically not present on muzzleloaders, as all those are single-shot firearms with a closed off breech with the powder and projectile manually loaded from the muzzle. Instead, the muzzleloader ignition mechanism is referred to as the lock.
A view of the break-action of a typical double-barreled shotgun, with the action open and the extractor visible. The opening lever and the safety catch are clearly visible.
Smith and Wesson M&P revolver
Barreled action for bolt-action rifle
U.S. M1895 Lee Navy straight-pull rifle