You mean a selective fire, magazine fed, rifle or carbine chambered for a medium powered round?
(Damn! I forgot the bayonet! damn, damn, damn, Oh! Sh!t! and the dreaded flash suppressor!)
I'm assuming you mean the term is misunderstood by the non-shooting public, media, etc which probably covers about 98% of us.
Exactly what I mean. Lot's of folks think an M14 or an FAL is an assault rifle. Or an M1A. Or a semi-auto CETME G3.
Actually, an M2 .30 carbine would meet the definition.