An automatic rifle is a rifle which is only capable of fully automatic fire. These are not usually select-fire weapons, and usually fire from an open bolt.
Often mistakenly defined as a "machine gun".
Machine guns are crew served weapons, meaning it generally takes 2 or more people to operate the weapon system.
An automatic rifle is issued to and used by an individual infantryman, usually in the place of the S.A.W. (Squad Automatic Weapon).
The M1917 Lewis gun, the M1918 B.A.R., and the French Sho-Sho of WWI were automatic rifles, used as light machine guns.