Automobile: 4-wheeled motor vehicle; usually propelled by an internal combustion engine (using volatile inflammable liquids, as gasoline or petrol, alcohol, naphtha, etc.), steam engines, or electric motors. Automobiles are also commonly, and generally in British usage, called motor cars.
Auto: An abbrev. of automobile, used as a prefix with the meaning of self-moving, self-propelling; as, an autocar, an autocarriage, an autotruck, etc.