The Collaborative International Dictionary of English v.0.48:

Obstruction \Ob*struc"tion\, n. [L. obstructio.]
   1. The act of obstructing, or state of being obstructed.
      [1913 Webster]

   2. That which obstructs or impedes; an obstacle; an
      impediment; a hindrance.
      [1913 Webster]

            A popular assembly free from obstruction. --Swift.
      [1913 Webster]

   3. The condition of having the natural powers obstructed in
      their usual course; the arrest of the vital functions;
      death. [Poetic]
      [1913 Webster]

            To die, and go we know not where,
            To lie in cold obstruction, and to rot. --Shak.
      [1913 Webster]

   Syn: Obstacle; bar; barrier; impediment; clog; check;
        hindrance.

   Usage: Obstruction, Obstacle. The difference between
          these words is that indicated by their etymology; an
          obstacle is something standing in the way; an
          obstruction is something put in the way. Obstacle
          implies more fixedness and is the stronger word. We
          remove obstructions; we surmount obstacles.
          [1913 Webster]

                Disparity in age seems a greater obstacle to an
                intimate friendship than inequality of fortune.
                                                  --Collier.
          [1913 Webster]

                The king expected to meet with all the
                obstructions and difficulties his enraged
                enemies could lay in his way.     --Clarendon.
          [1913 Webster]

WordNet (r) 3.0 (2006):

obstruction
    n 1: any structure that makes progress difficult [syn:
         obstruction, obstructor, obstructer, impediment,
         impedimenta]
    2: the physical condition of blocking or filling a passage with
       an obstruction [syn: obstruction, blockage]
    3: 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]
    4: the act of obstructing; "obstruction of justice"
    5: getting in someone's way

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "obstruction":
   afterthought, arrest, arrestation, arrestment, bar, barrier, bind,
   blank wall, blind alley, blind gut, block, blockade, blockage,
   blocking, bottleneck, bureaucratic delay, catch, cecum, cessation,
   check, checking, choking, choking off, clog, clogging, closing up,
   closure, congestion, constipation, constraint, constriction,
   cordon, costiveness, cramp, cul-de-sac, curtain, dead end,
   deceleration, delay, delayage, delayed reaction, detainment,
   detention, determent, deterrent, difficulty, double take, drag,
   dragging, drawback, ease-off, ease-up, embolism, embolus, fixation,
   flagging, foot-dragging, forbiddance, forbidding, gorge, halt,
   halting, hamper, hampering, hang-up, hazard, hindering, hindrance,
   hitch, holdback, holdup, hurdle, impasse, impediment, impeding,
   infarct, infarction, inhibition, interference, interim,
   interruption, jam, joker, lag, lagging, let, letdown, letup,
   limitation, limiting, logjam, minus acceleration, moratorium,
   mountain, negativism, nuisance value, objection, obstacle,
   obstipation, obstructionism, obstructive, occlusion,
   one small difficulty, opposition, paperasserie, pause,
   proscription, red tape, red-tapeism, red-tapery, repression,
   reprieve, resistance, respite, restraint, restriction, retardance,
   retardation, retardment, rub, sealing off, setback, slack-up,
   slackening, slow-up, slowdown, slowing, slowing down, slowness,
   slowup, snag, squeeze, stay, stay of execution, stop, stoppage,
   stopping, stranglehold, strangulation, stricture, stumbling block,
   stumbling stone, suppression, suspension, tie-up, time lag, wait