- A component, device, or unit with an input and an output, which can be inserted into a system without the need to know its internal structure. It is simply used based on its input and output characteristics. Also called closed box (1).
- In computers, a hardware or software unit whose internal structure is unknown, but whose function is known. Also called closed box (2).
- A sturdy self-contained electronic device which records key information concerning an aircraft, such as flight data.
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