A LoopHole is a way to escape/breach intended behavior or design.

For example if a programming language has a strong type system, one LoopHole may be to use pointers, which can be used to trick the language into being weakly typed even though it was intended to be strongly typed.  

Another example of a loophole is a security issue in an operating system; virus writers find loopholes to get into systems and damage them.