A transmission is a machine in a power transmission system that provides controlled power application. Often the term 5-speed transmission refers simply to the gearbox that uses gears and gear trains to provide speed and torque block conversions from a rotating power source to another device.

The term transmission properly refers to the whole drivetrain, including clutch, gearbox, prop shaft (for rear-wheel drive vehicles), differential, and final drive shafts. In the United States, the term is sometimes used in casual speech to refer more specifically to the gearbox alone, and detailed usage differs.

The most common use is in motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines must operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel. The transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process.

Transmissions are also used on pedal bicycles, fixed machines, and where different rotational speeds and torques are adapted. Often, a transmission has multiple gear ratios (or simply “gears”) and can switch between them as the rate varies. This switching may be done manually (by the operator) or automatically (by a control unit). Directional (forward and reverse) control may also be provided. Single-ratio transmissions also change the motor output speed and torque (and sometimes direction).

In motor vehicles, the transmission is generally connected to the engine crankshaft via a flywheel, clutch, or fluid coupling, partly because internal combustion engines cannot run below a particular speed. The transmission output is transmitted via the driveshaft to one or more differentials, which drive the wheels. While a differential may also provide gear reduction, its primary purpose is to permit the wheels at either end of an axle to rotate at different speeds (essential to avoid wheel slippage on turns) as it changes the direction of rotation.

Conventional gear/belt transmissions are not the only mpeed/torque adaptation mechanism. Alternative mechanisms include torque converters and power transformation (e.g., Diesel-electric transmission and hydraulic drive system).

Hybrid configurations also exist. Automatic transmissions use a valve body to shift gears using fluid pressures in response to engine RPM, speed, and throttle input.