An automatic firearm is a firearm

Firearm

A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

 that loads another round mechanically after the first round has been fired.

The term can be used to refer to semi-automatic firearm

Semi-automatic firearm

A semi-automatic, or self-loading firearm is a weapon which performs all steps necessary to prepare the weapon to fire again after firing—assuming cartridges remain in the weapon's feed device or magazine...

s, which fire one shot per single pull of the trigger (like the .45 "automatic"), or fully automatic firearms, which will continue to load and fire ammunition

Ammunition

Ammunition is a generic term derived from the French language la munition which embraced all material used for war , but which in time came to refer specifically to gunpowder and artillery. The collective term for all types of ammunition is munitions...

 until the trigger (or other activating device) is released, the ammunition is exhausted, or the firearm is jammed.
An automatic weapon fires, automatically extracts the used cartridge

Cartridge (firearms)

A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm. The primer is a small charge of impact-sensitive chemical that may be located at the center of the case head or at its rim . Electrically...

 case from the barrel and ejects it, then loads a new case into the barrel; generally either by harnessing the recoil of the cartridge's explosion, or the diversion of propellant gases.
An "automatic pistol

Pistol

When distinguished as a subset of handguns, a pistol is a handgun with a chamber that is integral with the barrel, as opposed to a revolver, wherein the chamber is separate from the barrel as a revolving cylinder. Typically, pistols have an effective range of about 100 feet.-History:The pistol...

" or "automatic shotgun

Shotgun

A shotgun is a firearm that is usually designed to be fired from the shoulder, which uses the energy of a fixed shell to fire a number of small spherical pellets called shot, or a solid projectile called a slug...

" generally refers to a semi-automatic design, though full-automatic pistols (e.g.