The Collaborative International Dictionary of English v.0.48:

Obstacle \Ob"sta*cle\, n. [F., fr. L. obstaculum, fr. obstare to
   withstand, oppose; ob (see Ob-) + stare to stand. See
   Stand. and cf. Oust, v.]
   That which stands in the way, or opposes; anything that
   hinders progress; a hindrance; an obstruction, physical or
   moral.
   [1913 Webster]

         If all obstacles were cut away.
         And that my path were even to the crown. --Shak.
   [1913 Webster]

   Syn: Impediment; obstuction; hindrance; difficulty. See
        Impediment, and Obstruction.
        [1913 Webster]

WordNet (r) 3.0 (2006):

obstacle
    n 1: something immaterial that stands in the way and must be
         circumvented or surmounted; "lack of imagination is an
         obstacle to one's advancement"; "the poverty of a district
         is an obstacle to good education"; "the filibuster was a
         major obstruction to the success of their plan" [syn:
         obstacle, obstruction]
    2: an obstruction that stands in the way (and must be removed or
       surmounted or circumvented)

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "obstacle":
   bar, barrier, blank wall, blind alley, blind gut, block, blockade,
   blockage, bottleneck, catch, cecum, check, choking, choking off,
   clog, congestion, constipation, cordon, costiveness, crimp,
   cul-de-sac, curtain, dead end, determent, deterrent, difficulty,
   disincentive, drawback, embolism, embolus, gorge, hamper, hang-up,
   hazard, hindrance, hitch, hurdle, impasse, impediment, infarct,
   infarction, jam, joker, mountain, objection, obstipation,
   obstruction, obstructive, one small difficulty, rub, sealing off,
   snag, stop, stoppage, strangulation, stumbling, stumbling block,
   stumbling stone