1. Any electrical device; such as, a switch or a potentiometer (a resistor with three terminals, the third being an adjustable center terminal; used to adjust voltages in radios and TV sets), which is used to vary a circuit parameter.
2. The control of a machine by electric switches, relays, rheostats, or a resistor designed to allow variation in resistance without breaking the electrical circuit of which it is a part.
3. The control of a machine or instrument by switches, relays, or rheostats, as contrasted with electronic control by electron tubes or by devices which do the work of electron tubes.