The term assault rifle is generally attributed to Adolf Hitler, who for propaganda purposes used the German word "Sturmgewehr" (which translates to "assault rifle"), as the new name for the MP43, subsequently known as the Sturmgewehr 44 or StG 44. Other sources dispute that Hitler had much to do with coining the new name besides signing the production order. The StG 44 is generally considered the first selective fire military rifle to popularize the assault rifle concept. Today, the term assault rifle is used to define firearms sharing the same basic characteristics as the StG 44
In a strict definition, a firearm must have at least the following characteristics to be considered an assault rifle
It must be an individual weapon
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 box magazine
And it should have an effective range of at least 300 metres (330 yards)
Rifles that meet most of these criteria, but not all, are technically not assault rifles despite frequently being called such.
For example:
Select-fire M2 Carbines are not assault rifles; their effective range is only 200 meters.
Select-fire rifles such as the FN FAL battle rifle are not assault rifles; they fire full-powered rifle cartridges.
Semi-automatic-only rifles like variants of the Colt AR-15 are not assault rifles; they do not have select-fire capabilities.
https://en.wikipedia.org/wiki/Assault_rifle