An automatic weapon is a ranged weapon (be it a fictional directed-energy weapon or real-life firearm) that fires more than one shot when the user pulls the trigger. A fully automatic weapon can fire indefinitely on a single trigger pull, stopping only when its mechanism jams or it runs out of ammunition.
While an automatic weapon can also be called a machine gun the terms are not interchangeable. For example, a Gatling Gun is also a type of machine gun, using machinery to fire multiple rounds, but it requires the operator to crank the mechanism in order for the gun to continue to fire. Most automatic weapons use the energy produced by the fired round to power the weapon loading and firing the next round. There are two major system used in actuating the machinery, gas and recoil operation. Gas operated machine guns use the pressure of the discharged round to push a piston that actuates the weapon while recoil systems use the kinetic energy of the round.
The first fully automatic weapon was developed by Hiram Stevens Maxim in 1884. The advent of automatic weapons had a major effect on tactics, rendering massed charges obsolete, as was demonstrated at the Battle of Omdurman in 1889, in which ten British Maxims decimated over 15,000 cavalrymen. Later, during the Great War, small numbers of machine gun emplacements were able to fend off tens of thousands of soldiers. In the first half of the 20th century, automatic weapons were minaturized to serve as basic infantry weapons, starting with the development of submachine gun (the German MP-18 being the first) and eventually to the development of the assault rifle. Another famous automatic weapons designer is John M. Browning, he being one of the most prolific in history, creating some of the most successful automatic weapons ever designed including the M2 Heavy Machine Gun, M1917 Water-Cooled machine gun, M1919 Air-Cooled machine gun and numerous others.