NOUN (2)

1. an ambiguity (especially one in the text of a law or contract) that makes it possible to evade a difficulty or obligation;

2. a small hole in a fortified wall; for observation or discharging weapons;

The Collaborative International Dictionary of English v.0.48:

Loophole \Loop"hole`\, n.
   1. (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.
      [1913 Webster]

   2. A hole or aperture that gives a passage, or the means of
      escape or evasion.
      [1913 Webster]

   3. 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; as, a loophole in the law big enough to
      drive a truck through.
      [PJC]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

146 Moby Thesaurus words for "loophole":
   Texas tower, abatis, advanced work, air hole, alternative, armhole,
   avenue, balistraria, bank, banquette, barbed-wire entanglement,
   barbican, barricade, barrier, bartizan, bastion, battlement,
   beacon, belvedere, bleachers, blowhole, breastwork, bridge,
   bullet-hole, bulwark, bunghole, casemate, channel, cheval-de-frise,
   choice, chute, circumvallation, conning tower, contravallation,
   counterscarp, cringle, curtain, deadeye, debouch, demibastion,
   dike, dodge, door, drawbridge, earthwork, egress, emunctory,
   enclosure, entanglement, escape, escape clause, escape hatch,
   escarp, escarpment, estuary, evasion, exhaust, exit, eye, eyelet,
   fence, fieldwork, floodgate, flume, fortalice, fortification,
   gallery, gasket, gazebo, glacis, grandstand, grommet, guide,
   keyhole, knothole, lighthouse, lookout, loop, lunette,
   machicolation, manhole, mantelet, merlon, mound, mousehole,
   observation post, observatory, opening, out, outcome, outfall,
   outgate, outgo, outlet, outlook, outwork, overlook, palisade,
   parados, parapet, peanut gallery, peephole, pharos, pigeonhole,
   pinhole, placket, placket hole, pore, port, portcullis, porthole,
   postern gate, pretext, punch-hole, quibble, rampart, ravelin,
   redan, redoubt, ringside, ringside seat, sally port, saving clause,
   scarp, sconce, sighthole, sluice, spiracle, spout, stockade,
   subterfuge, tap, tenaille, top gallery, tower, vallation, vallum,
   vent, ventage, venthole, vomitory, watchtower, way of escape,
   way out, weir, work