The Collaborative International Dictionary of English v.0.48:

Impasse \Im`passe"\ ([a^]N`p[aum]s"; E. [i^]m*p[.a]s"), n. [F.]
   An impassable road or way; a blind alley; cul-de-sac; fig., a
   position or predicament affording no escape.

         The issue from the present impasse will, in all
         probability, proceed from below, not from above.
                                                  --Arnold
                                                  White.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

impasse
    n 1: a situation in which no progress can be made or no
         advancement is possible; "reached an impasse on the
         negotiations" [syn: deadlock, dead end, impasse,
         stalemate, standstill]
    2: a street with only one way in or out [syn: blind alley,
       cul de sac, dead-end street, impasse]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "impasse":
   bar, barrier, blank wall, blind alley, blind gut, block, blockade,
   blockage, bottleneck, box, cecum, choking, choking off, clog,
   congestion, constipation, corner, costiveness, cul-de-sac,
   dead end, dead-end street, deadlock, dilemma, embolism, embolus,
   extremity, fix, gorge, halt, hole, impediment, infarct, infarction,
   jam, obstacle, obstipation, obstruction, pickle, plight, pocket,
   scrape, sealing off, stalemate, stand, stand-off, standstill, stop,
   stoppage, strangulation