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. Aeron.an electronic device, such as a flight recorder, that can be removed from an aircraft as a single package.
4. Auto.a device in an electronic ignition system that generates electrical pulses.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.