Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

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

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

   4. An any obstruction; anything which hinders approach or
      attack. ``Constitutional barriers.'' --Hopkinson.

   5. Any limit or boundary; a line of separation.

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

   {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.]

Source : WordNet®

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