From wikipedia:
The term assault rifle is a translation of the German word Sturmgewehr (literally meaning "storm rifle"), "storm" used as a verb being synonymous with assault, as in "to storm the compound". Sturmgewehr was coined by Adolf Hitler [citation needed] to describe the Maschinenpistole 44, subsequently re-christened Sturmgewehr 44, the firearm generally considered the first true assault rifle and served to popularize the concept. The translation “assault rifle” gradually became the common term for similar firearms sharing the same technical definition as the name giver StG 44. In a strict definition, a firearm must have at least the following characteristics to be considered an assault rifle:[1][2][3]
*A carbine sized individual weapon with provision to fire from the shoulder.
*Capable of selective fire.
*Intermediate-power cartridge between pistol and traditional rifle.
*Ammunition is supplied from a large capacity detachable box magazine.
The following features are commonly found on assault rifles, but those are not exclusive to assault rifles, as those features are shared with many submachine guns, battle rifles, automatic rifles, machine guns, and semi-automatic rifles:
*Protruding pistol grip.
*Muzzle device like a muzzle brake or a flash suppressor.
There are commentators who use the expression “assault rifle” more loosely to include other types of arms, particularly arms that fall under a strict definition of the battle rifle, or civilian semi-automatic off-shoots of military rifles for commercial or political reasons. Some militaries of nations outside of the English-speaking world also have a different definition of assault rifle. For instance, the analogous term in the Swedish Armed Forces is automatkarbin (literally "automatic carbine") which includes both assault rifles and battle rifles.
References
1. C. Taylor The fighting rifle – A complete study of the rifle in combat, ISBN 0-87947-308-8
2. F.A. Moyer Special Forces foreign weapons handbook, ISBN 0-87364-009-8
3. R.J. Scroggie, F.A. Moyer Special Forces combat firing techniques, ISBN 0-87364-010-1