1. A circuit which contains active electric parts; such as, electron tubes, electron transistors, and magnetic amplifiers, etc., as opposed to a circuit that contains only passive components; such as, resistors and switches.
2. An electric circuit in which the balance of electrons in a given electric part; such as, a tube, transistor, or amplifier, is disturbed by something other than an applied electric voltage.
3. An electric circuit having at least one element that manipulates the voltage or current in the circuit.