a device which does something. It is nearly always a source of emf, but its important property could be that it is sending a signal of some kind. (See also passive device.)
Devices such as transistors, integrated circuits and vacuum tubes that require an external power source in order to perform an intended purpose, such as amplification.
A component that has gain or operates in a non-linear fashion to change the basic character of an electric signal by, for example, amplification or rectification, or a device comprised of such component, for example a transistor switch. A device whose output is dependent upon an external source of power other than the input signal.