In science, computing, and engineering, a black box is a device, system or object which can be viewed in terms of its inputs and outputs (or 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 brain. The complementary term is a white or glass box, whose internals are open to inspection, and the pairing marks a choice of analytical stance rather than a property of the object — the same system is a black box or not depending on what the observer is permitted, or able, to see.
The term spread through World War II radar research; the historian Peter Galison traces its popularity to MIT's Radiation Laboratory, where electronic assemblies were housed in dark enclosures, and an alternative account points to the 1940 Tizard Mission, which carried an experimental cavity magnetron from Britain to the United States in a black metal box. Its theoretical form came from 📝Norbert Wiener's early-1940s work on antiaircraft prediction, which modelled the enemy pilot as a system to be predicted from behaviour rather than understood from construction. W. Ross Ashby formalized the method in An Introduction to Cybernetics (1956), devoting a chapter to it and arguing that all systems are ultimately black boxes, since complete internal knowledge is never available.
The stance is now standard practice in several fields. Black-box testing verifies that specified inputs produce expected outputs without reading the source code; cryptography and physics both use it to reason about systems whose internals are unknown or irrelevant. Its sharpest contemporary application is to 📝Artificial Intelligence (AI): the weights of a trained neural network are fully inspectable, yet the procedure by which they reach a given output is not legible to a human reader, which makes the system opaque in practice despite being open in principle. That gap is the motivating problem for interpretability and explainability research, and the reason "black box" now carries a governance charge it did not have in 1956.