Esperanto/Key Concepts/Black Box
The term "black box" is a convenient and figurative expression to portray a piece of apparatus which converts an input into an output. (Weiner 1948). It is useful when discussing a system at a certain level of abstraction at which the details of what goes on "under the bonnet" - i.e the actual mechanisms within the black box would only confuse the discussion.
The child who tries to open a door has to manipulate the handle (the input) so as to produce the desired movement at the latch (the output); and he has to learn how to control the one by the other without being able to see the internal mechanism that links them. In our daily lives we are confronted at every turn with systems whose internal mechanisms are not fully open to inspection, and which must be treated by the methods appropriate to the Black Box.
— W. Ross Ashby, An Introduction to Cybernetics, 1956