The Collaborative International Dictionary of English v.0.48:

Obstruct \Ob*struct"\, v. t. [imp. & p. p. Obstructed; p. pr.
   & vb. n. Obstructing.] [L. obstructus, p. p. of obstruere
   to build up before or against, to obstruct; ob (see Ob-) +
   struere to pile up. See Structure.]
   [1913 Webster]
   1. To block up; to stop up or close, as a way or passage; to
      place an obstacle in, or fill with obstacles or
      impediments that prevent or hinder passing; as, to
      obstruct a street; to obstruct the channels of the body.
      [1913 Webster]

            'T is the obstructed paths of sound shall clear.
                                                  --Pope.
      [1913 Webster]

   2. To be, or come, in the way of; to hinder from passing; to
      stop; to impede; to retard; as, the bar in the harbor
      obstructs the passage of ships; clouds obstruct the light
      of the sun; unwise rules obstruct legislation. "Th'
      impatience of obstructed love." --Johnson.
      [1913 Webster]

   Syn: To bar; barricade; stop; arrest; check; interrupt; clog;
        choke; impede; retard; embarrass; oppose.
        [1913 Webster]

WordNet (r) 3.0 (2006):

obstruct
    v 1: hinder or prevent the progress or accomplishment of; "His
         brother blocked him at every turn" [syn: obstruct,
         blockade, block, hinder, stymie, stymy,
         embarrass]
    2: block passage through; "obstruct the path" [syn: obstruct,
       obturate, impede, occlude, jam, block, close up]
       [ant: disengage, free]
    3: shut out from view or get in the way so as to hide from
       sight; "The thick curtain blocked the action on the stage";
       "The trees obstruct my view of the mountains" [syn:
       obstruct, block]