A pump-action rifle or shotgun is one in which the handgrip can be pumped back and forth in order to eject a spent round of ammunition and to chamber a fresh one. It is much faster than a bolt-action and somewhat faster than a lever-action, as it does not require the trigger hand to be removed from the trigger whilst reloading. When used in rifles, this action is also commonly called a slide action. The 40mm M203 grenade launcher mounted under the barrel of M16 rifles and M4 carbines is sometimes described as "pump or slide-action." It is not. Once fired, the barrel is slid forward by hand and the expended cartridge ejected. It is then hand reloaded by manually inserting a cartridge into the chamber and the barrel slid to the rear to arm the weapon. The reloading process does not involve mechanically reloading from a magazine.
Pump-action shotguns, also called "slide-action repeating shotguns" or "slide-action shotguns" are a class of shotguns that are distinguished in the way in which spent shells are extracted and fresh ones are chambered. The weapon has a single barrel above a tube magazine into which shells are inserted. New shells are chambered by pulling a pump handle (often called the fore-end) attached to the tube magazine toward the user, then pushing it back into place to chamber the cartridge (in a few cases this action is reversed). Fore-ends are replaceable, and modern ones may include a pistol grip for a more secure hold, picatinny rails, or a tactical light.
The term pump-action can also be applied to various airsoft guns and air guns, which use a similar mechanism to both load a pellet and compress a spring piston for power, or pneumatic guns where a pump is used to compress the air used for power. See the airgun article for information on how spring piston and pneumatic airguns work.