An autocannon is a projectile weapon with a larger bore size than a machine gun, and a faster rate of fire than regular cannon used by artillery units. The term developed from WWII guns used in aircraft, where the distinction was that the bullets were explosive. After the war similar guns were used with non-explosive rounds in the anti-tank role, and the name autocannon started to become popular. Autocannon today are typically distinquished by their incorporation of some method of automated loading and firing.
Examples of an autocannon are the 25 mm M242 Bushmaster mounted on the M2 Bradley, the Bofors 40 mm anti-aircraft gun, or the 30 mm GAU-8 in the A-10 Warthog close air support airframe.
Several tank guns have auto loading systems, and can be considered autocannon even though they are large-bore, up to 125 mm. Soviet and Russian armor, especially their main battle tanks, have used autoloaders extensively for many tank generations, even as US tanks keep human gun loaders. Typically, these are not commonly referred to as autocannon, but instead as tank guns.