Automatic transmission
An automatic transmission (also called automatic gearbox) is one type of motor vehicle
Motor vehicle
A motor vehicle or road vehicle is a self-propelled wheeled vehicle that does not operate on rails, such as trains or trolleys. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid...

Transmission (mechanics)
A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...

 that can automatically change gear ratio
Gear ratio
The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...

s as the vehicle moves, freeing the driver from having to shift gears manually
Manual transmission
A manual transmission, also known as a manual gearbox or standard transmission is a type of transmission used in motor vehicle applications...

. Most automatic transmissions have a defined set of gear ranges, often with a parking pawl
Parking pawl
A parking pawl is a device fitted to a motor vehicle's automatic transmission in order for it to lock up the transmission. It is engaged when the transmission shift lever selector is placed in the Park position, which is always the first position in all cars sold in the United States since 1965 A...

 feature that locks the output shaft of the transmission.

Similar but larger devices are also used for heavy-duty commercial and industrial vehicles and equipment.