black box

noun

1. any unit that forms part of an electronic circuit and that has its function, but not its components, specified.
2. any comparatively small, usually black, box containing a secret, mysterious, or complex mechanical or electronic device.
3. Aeronautics. an electronic device, such as a flight recorder, that can be removed from an aircraft as a single package.
4. Automotive. a device in an electronic ignition system that generates electrical pulses.

noun

1. a self-contained unit in an electronic or computer system whose circuitry need not be known to understand its function
2. an informal name for flight recorder