In modern usage, a torque converter is generally a type of hydrodynamic fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. The torque converter normally takes the place of a mechanical clutch in a vehicle with an automatic transmission, allowing the load to be separated from the power source. It is usually located between the engine's flywheel and the transmission.
The key characteristic of a torque converter is its ability to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. Some of these devices are also equipped with a temporary locking mechanism which rigidly binds the engine to the transmission when their speeds are nearly equal, to avoid slippage and a resulting loss of efficiency.
By far the most common form of torque converter in automobile transmissions is the device described here. However, in the 1920s there was also the pendulum-based Constantinesco torque converter. There are also mechanical designs for continuously variable transmissions and these also have the ability to multiply torque, e.g. the Variomatic with expanding pulleys and a belt drive.