loophole

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A way of escaping a difficulty, especially an omission or ambiguity in the wording of a contract or law that provides a means of evading compliance.
2. n. A small hole or slit in a wall, especially one through which small arms may be fired.

Century Dictionary and Cyclopedia

1. n. A small aperture, narrow toward the outside and splayed within, in the walls of a fortification or of any similar structure, through which small-arms may be fired at an enemy, or observations may be taken.
2. n. An opening into or out of anything; a hole or aperture that gives a passage or the means of escape: often used figuratively, and especially of an underhand or unfair method of escape or evasion.

Wiktionary

1. n. A method of escape, especially an ambiguity or exception in a rule that can be exploited in order to avoid its effect.
2. n. A slit in a castle wall. Later: any similar window for shooting a weapon or letting in light.
3. v. military To prepare a building for defense by preparing slits or holes through which to fire on attackers

GNU Webster's 1913

1. n. (Mil.) A small opening, as in the walls of fortification, or in the bulkhead of a ship, through which small arms or other weapons may be discharged at an enemy.
2. n. A hole or aperture that gives a passage, or the means of escape or evasion.
3. n. An amibiguity or unintended omission in a law, rule, regulation, or contract which allows a party to circumvent the intent of the text and avoid its obligations under certain circumstances. -- used usually in a negative sense; -- distinguished from escape clause in that the latter usually is included to deliberately allow evasion of obligation under certain specified and foreseen circumstances.

WordNet 3.0

1. n. a small hole in a fortified wall; for observation or discharging weapons
2. n. an ambiguity (especially one in the text of a law or contract) that makes it possible to evade a difficulty or obligation