White box (software engineering)
A white box is a subsystem whose internals can be viewed but usually not altered.
Having access to the subsystem internals in general makes the subsystem easier to understand but also easier to hack; for example, if a programmer can examine source code, weaknesses in an algorithm are much easier to discover. That makes white-box testing much more effective than black-box testing but considerably more difficult from the sophistication needed on the part of the tester to understand the subsystem.
