The term is used by people with political ambitions or by "journalists" with agendas. Basically it is a term used to scare folks. It is common place for the "firearm uneducated" to use it thinking its actually a standard or correct term.
IMO it is not a correct term, and should not be a standard term. Depending on who you ask, an assault weapon can be anything that's auto-loading (semi-auto), full auto, select fire, that uses an external magazine, can carry more than XXXX bullets, has a bayonet, adjustable stock, folding stock, a military or large caliber, or for some reason just looks scary to the speaker.
Look up the term "assault", it can be a verb or a noun but NOT an adjective.
p.s. AR stands for Armalite, NOT "assault rifle".