White box (software engineering)

A white box (or glass box, clear box, or open box) is a subsystem whose internals can be viewed but usually not be 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.