Our friends at wikipedia have the following to say: (Red Highlights by ME)
The term assault rifle is a translation of the German word Sturmgewehr (literally meaning "storm rifle"), "storm" used synonymous with assault. The name was coined by Adolf Hitler[1] to describe the Maschinenpistole 44, subsequently re-christened Sturmgewehr 44, the firearm generally considered the first true assault rifle that served to popularize the concept.
The translation assault rifle gradually became the common term for similar firearms sharing the same technical definition as the StG 44. In a strict definition, a firearm must have at least the following characteristics to be considered an assault rifle:[2][3][4]
It must be an individual weapon with provision to fire from the shoulder (i.e. a buttstock);
It must be capable of selective fire; It must have an intermediate-power cartridge: more power than a pistol but less than a standard rifle or battle rifle;
Its ammunition must be supplied from a detachable magazine.
Rifles that meet most of these criteria, but not all, are technically not assault rifles despite frequently being considered as such.
For example, semi-automatic-only rifles that share designs with assault rifles such as the AR-15 (which the M-16 rifle is based on) are not assault rifles, as they are not capable of switching to automatic fire and thus not selective fire. Belt-fed weapons (such as the M249 SAW) or rifles with fixed magazines are likewise not assault rifles because they do not have detachable box magazines.
The term "assault rifle" is often more loosely used for commercial or political reasons to include other types of arms, particularly arms that fall under a strict definition of the battle rifle, or semi-automatic variant of military rifles such as AR-15s.
The US Army defines assault rifles as "short, compact, selective-fire weapons that fire a cartridge intermediate in power between submachinegun and rifle cartridges
