black box

A black box is a unit of which the transmission properties are known, but not its internal functioning.

A black box is a device or a system component with known properties, which are expressed in the fact that the output signal has a functional relationship to the input signal. The inner function may be a logical function, but it may also be an analog one, expressed in terms of gain, frequency response or impulse response, or other parameters.

The knowledge about the inner function, about its structure or mode of operation, is not of interest for the use of black boxes, because they are considered as analog or digital functional units.