Black Box

A black box is any kind of device that has a simple, well defined interface and that performs some discrete, well defined function. A black box is so called because you don't need to see what is going on inside of it. All you need to know is what it does, what its inputs are, and sometimes what its outputs are. We encounter black boxes every day. A wrist watch is a good example. A typical watch has some buttons or dials with which you can set the time (the inputs), and a face that you can read to determine the time at any given moment (the out puts). You don't need to know or care how all the gears and gizmos inside the watch are put together in order for the watch to be useful to you. Unless you are an aficionado or collector of watches, you don't really care if it has quartz movement or if there's a small rodent running around inside to keep the watch ticking. It's a black box. All that's important to you are that it works, and that you understand its inputs and outputs.