Black box is a technical term for a device or system or object when it is viewed primarily in terms of its input and output characteristics. Almost anything might occasionally be referred to as a black box: a transistor, an algorithm, humans, the Internet.

The opposite of a black box is a system where the inner components or logic are available for inspection (such as a free software/open source program) which is sometimes known as a white box, a glass box, or a clear box. Points of view such as interactive computation may see the concept of a black box as a useful fiction.

Common usage

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.Black-Box Testing: Techniques for Functional Testing of Software and Systems, by Boris Beizer, 1995. ISBN 0471120944 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.

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.Cybernetics: Or the Control and Communication in the Animal and the Machine, by Norbert Wiener, page xi, MIT Press, 1961, ISBN 026273009X 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 be tested by the programmers.

In the stock market many people trade with "Black box" programs and algorithms designed by programmers.Breaking the Black Box, by Martin J. Pring, McGraw-Hill, 2002, ISBN 0071384057 These programs automatically trade user's 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. Sometimes black box is used as a synonym for black body.

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''."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

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.

In Theatre, a black box theatre is a theatre that the seating area and the stage can be changed into whatever the director wishes.