An automatic firearm is a firearm that will continue to load and fire rounds of ammunition as long as the trigger (or equivalent) is activated or until it runs out of ammunition.
The .50 caliber (12.7 mm) M2 machine gun, the 7.62 mm NATO M60 machine gun and the M134 minigun are examples of fully-automatic weapons, as are the autocannon mounted on any number of military aircraft and used in other military applications for point defense and/or anti-aircraft defense.
Many other weapons (submachine guns, assault rifles, machine pistols) are capable of fully-automatic fire, but most also have a selective fire capability.
So-called automatic pistols, however, are generally not fully automatic weapons, but fire a single shot every time that the trigger is pulled, automatically ejecting the spent cartridge and loading a fresh one into the chamber.