A
device used in
electric rc cars to control the rate at which the
electric motor operates. It's largely
replaced mechanical speed control. Commonly
abbreviated to ESC.
The inputs to the are car's battery, and a connection from the receiver. The output is a couple of wires to the electric motor. The receiver connection is plugged in to where the throttle servo (if you were using a mechanical speed control or a nitro engine) usually goes. It takes this information from the receiver and uses it to control the amount of voltage going to the engine.
ESC's employ a technique very similar to a switch mode power supply. They take the input voltage from the battery, and switch it on and off at the appropriate rate so that averaged over time, the voltage would be equal to the desired voltage. For example say you had a 7.2 volt battery and only wanted to send 3.6 volts to the motor. Then you'd allow the full 7.2 volts for 50 per cent of the time, and 0 volts for 50 per cent of the time; giving an average of 3.6 volts. You'd probably use a capacitor to smooth the signal out. This makes them much more efficient than resistor-based techniques used in mechanical speed controls, and also makes them truly proportional (i.e., you can control the motor to have any voltage you like between 0 and the battery voltage).
Because they are expensive, many kits include the mechanical speed control and you have to buy the ESC as an upgrade. Well-known ESC manufacturers inlcude Novak and Tekin.