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.
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   2. A fortress or fortified town, on the frontier of a
      country, commanding an avenue of approach.
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   3. pl. A fence or railing to mark the limits of a place, or
      to keep back a crowd.
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            No sooner were the barriers opened, than he paced
            into the lists.                       --Sir W.
                                                  Scott.
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   4. Any obstruction; anything which hinders approach or
      attack. "Constitutional barriers." --Hopkinson.
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   5. Any limit or boundary; a line of separation.
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            'Twixt that [instinct] and reason, what a nice
            barrier!                              --Pope.
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   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.]
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WordNet (r) 3.0 (2006):

barrier reef
    n 1: a long coral reef near and parallel to the shore