In science and engineering, a black box is a device, system or object which can be viewed in terms of its input, output and transfer characteristics without any knowledge of its internal workings. Its implementation is "opaque" (black). Almost anything might be referred to as a black box: a transistor, an algorithm, or the human mind.
The opposite of a black box is a system where the inner components or logic are available for inspection, which is sometimes known as a clear box, a glass box, or a white box.
The modern term "black box" seems to have entered the English language around 1945. The process of network synthesis from the transfer functions of black boxes can be traced to Wilhelm Cauer who published his ideas in their most developed form in 1941. Although Cauer did not himself use the term, others who followed him certainly did describe the method as black-box analysis. Vitold Belevitch puts the concept of black-boxes even earlier, attributing the explicit use of two-port networks as black boxes to Franz Breisig in 1921 and argues that 2-terminal components were implicitly treated as black-boxes before that.
- In electronics, a sealed piece of replaceable equipment; see line-replaceable unit (LRU).
- In computer programming and software engineering, black box testing is used to check that the output of a program is as expected, given certain inputs. The term "black box" is used because the actual program being executed is not examined.
- In computing in general, a black box program is one where the user cannot see its inner workings (perhaps because it is a closed source program) or one which has no side effects and the function of which need not be examined, a routine suitable for re-use.
- Also in computing, a black box refers to a piece of equipment provided by a vendor, for the purpose of using that vendor's product. It is often the case that the vendor maintains and supports this equipment, and the company receiving the black box typically are hands-off.
- In cybernetics a black box was described by Norbert Wiener as an unknown system that was to be identified using the techniques of system identification. He saw the first step in Self-organization as being to be able to copy the output behaviour of a black box.
- In neural networking or heuristic algorithms (computer terms generally used to describe 'learning' computers or 'AI simulations') a black box is used to describe the constantly changing section of the program environment which cannot easily be tested by the programmers. This is also called a White box (software engineering) in the context that the program code can be seen, but the code is so complex that it might as well be a Black box.
- In finance many people trade with "black box" programs and algorithms designed by programmers. These programs automatically trade users' accounts when certain technical market conditions suddenly exist (such as a SMA crossover).
- In physics, a black box is a system whose internal structure is unknown, or need not be considered for a particular purpose.
- In mathematical modelling, a limiting case.
- In philosophy and psychology, the school of behaviorism sees the human mind as a black box; see black box theory.
- In neorealist international relations theory, the sovereign state is generally considered a black box: states are assumed to be unitary, rational, self-interested actors, and the actual decision-making processes of the state are disregarded as being largely irrelevant. Liberal and constructivist theorists often criticize neorealism for the "black box" model, and refer to much of their work on how states arrive at decisions as "breaking open the black box".
- In cryptography to capture the notion of knowledge obtained by an algorithm through the execution of a cryptographic protocol such as a zero-knowledge proof protocol. If the output of the algorithm when interacting with the protocol can be simulated by a simulator that interacts only the algorithm, this means that the algorithm 'cannot know' anything more than the input of the simulator. If the simulator can only interact with the algorithm in a black box way, we speak of a black box simulator.
Other uses of the term 
- In aviation, a "black box" (they are actually bright orange, to facilitate their being found after a crash) is an audio or data recording device in an airplane or helicopter. The cockpit voice recorder records the conversation of the pilots and the flight data recorder logs information about controls and sensors, so that in the event of an accident investigators can use the recordings to assist in the investigation. Although these devices were originally called black boxes for a different reason, they are also an example of a black box according to the meaning above, in that it is of no concern how the recording is actually made.
- In amateur radio the term "black box operator" is a disparaging or self-deprecating description of someone who operates factory made radios without having a good understanding of how they work. Such operators don't build their own equipment (an activity called "homebrewing") or repair their own "black boxes".
See also 
- W. Cauer. Theorie der linearen Wechselstromschaltungen, Vol.I. Akad. Verlags-Gesellschaft Becker und Erler, Leipzig, 1941.
- E. Cauer, W. Mathis, and R. Pauli, "Life and Work of Wilhelm Cauer (1900 – 1945)", Proceedings of the Fourteenth International Symposium of Mathematical Theory of Networks and Systems (MTNS2000), p4, Perpignan, June, 2000. Retrieved online 19th September 2008.
- Belevitch, V, "Summary of the history of circuit theory", Proceedings of the IRE, vol 50, Iss 5, pp848-855, May 1962.
- Black-Box Testing: Techniques for Functional Testing of Software and Systems, by Boris Beizer, 1995. ISBN 0-471-12094-4
- Cybernetics: Or the Control and Communication in the Animal and the Machine, by Norbert Wiener, page xi, MIT Press, 1961, ISBN 0-262-73009-X
- Breaking the Black Box, by Martin J. Pring, McGraw-Hill, 2002, ISBN 0-07-138405-7
- "Mind as a Black Box: The Behaviorist Approach", pp 85-88, in Cognitive Science: An Introduction to the Study of Mind, by Jay Friedenberg, Gordon Silverman, Sage Publications, 2006