loophole

Short definition from Wiktionary

Noun

1. 1
   A slit in a castle wall; today, any similar window for shooting a ranged weapon or letting in light.
2. 2
   A method of escape, especially an ambiguity or exception in a rule or law that can be exploited in order to avoid its effect.

Verb

1. 1
   To prepare a building for defense by preparing slits or holes through which to fire on attackers
2. 2
   To exploit (a law, etc.) by means of loopholes.