WordNet (r) 3.0 (2006):

escape hatch
    n 1: hatchway that provides a means of escape in an emergency

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "escape hatch":
   alternative, back door, back stairs, back way, bolt-hole,
   boundary condition, catch, choice, clause, condition, covert way,
   donnee, ejection seat, emergency exit, escalator clause,
   escalier derobe, escape clause, escape route, fine print,
   fire escape, given, grounds, inflatable slide, joker, kicker,
   life buoy, life net, life raft, lifeboat, lifeline,
   limiting condition, loophole, obligation, parameter, prerequisite,
   pretext, provision, provisions, proviso, requisite, sally port,
   saving clause, secret exit, secret passage, secret staircase,
   side door, sine qua non, slide, small print, specification,
   stipulation, string, terms, ultimatum, underground,
   underground railroad, underground route, way of escape, way out,
   whereas