In modern usage, a torque converter is generally a type of fluid coupling (but also being able to multiply torque) 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 usually takes the place of a mechanical clutch in a vehicle with an automatic transmission, separating the load from the power source. It is generally located between the engine’s flexplate and the transmission.
The essential 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 that rigidly binds the engine to the transmission when their speeds are nearly equal to avoid slippage and a resulting loss of efficiency.