The Collaborative International Dictionary of English v.0.48:

Barrier \Bar"ri*er\, n. [OE. barrere, barere, F. barri[`e]re,
   fr. barre bar. See Bar, n.]
   1. (Fort.) A carpentry obstruction, stockade, or other
      obstacle made in a passage in order to stop an enemy.
      [1913 Webster]

   2. A fortress or fortified town, on the frontier of a
      country, commanding an avenue of approach.
      [1913 Webster]

   3. pl. A fence or railing to mark the limits of a place, or
      to keep back a crowd.
      [1913 Webster]

            No sooner were the barriers opened, than he paced
            into the lists.                       --Sir W.
                                                  Scott.
      [1913 Webster]

   4. Any obstruction; anything which hinders approach or
      attack. "Constitutional barriers." --Hopkinson.
      [1913 Webster]

   5. Any limit or boundary; a line of separation.
      [1913 Webster]

            'Twixt that [instinct] and reason, what a nice
            barrier!                              --Pope.
      [1913 Webster]

   Barrier gate, a heavy gate to close the opening through a
      barrier.

   Barrier reef, a form of coral reef which runs in the
      general direction of the shore, and incloses a lagoon
      channel more or less extensive.

   To fight at barriers, to fight with a barrier between, as a
      martial exercise. [Obs.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

barrier
    n 1: a structure or object that impedes free movement
    2: any condition that makes it difficult to make progress or to
       achieve an objective; "intolerance is a barrier to
       understanding" [syn: barrier, roadblock]
    3: anything serving to maintain separation by obstructing vision
       or access

Moby Thesaurus II by Grady Ward, 1.0:

161 Moby Thesaurus words for "barrier":
   abatis, advanced work, arch dam, backstop, balistraria, balustrade,
   bamboo curtain, bank, banquette, bar, barbed-wire entanglement,
   barbican, barrage, barricade, bartizan, bastion, battlement,
   bear-trap dam, beaver dam, blank wall, blind alley, blind gut,
   block, blockade, blockage, bolt, boom, bottleneck, boundary,
   brattice, breakwater, breastwork, brick wall, buffer, buffer state,
   bulkhead, bulwark, bumper, casemate, cecum, cheval-de-frise,
   choking, choking off, circumvallation, clog, cloison, cofferdam,
   collision mat, congestion, constipation, contravallation,
   costiveness, counterscarp, cul-de-sac, curtain, cushion, dam,
   dead end, defense, demibastion, diaphragm, dike, dissepiment,
   ditch, dividing line, dividing wall, division, drawbridge,
   earthwork, embankment, embolism, embolus, enclosure, entanglement,
   escarp, escarpment, fence, fender, fieldwork, fortalice,
   fortification, frontier, gate, glacis, gorge, gravity dam, groin,
   ha-ha, hindrance, hydraulic-fill dam, impasse, impediment, infarct,
   infarction, interseptum, iron curtain, jam, jetty, leaping weir,
   levee, limit, lock, logjam, loophole, lunette, machicolation,
   mantelet, mat, merlon, midriff, midsection, milldam, moat, mole,
   mound, obstacle, obstipation, obstruction, outwork, pad, padlock,
   palisade, panel, parados, parapet, paries, partition, party wall,
   portcullis, postern gate, property line, rail, railing, rampart,
   ravelin, redan, redoubt, roadblock, rock-fill dam, sally port,
   scarp, sconce, sealing off, seawall, separation, septulum, septum,
   shock pad, shutter dam, stockade, stone wall, stop, stoppage,
   strangulation, tenaille, vallation, vallum, wall, weir, wicket dam,
   work