Barrier

Bar·ri·er

Barrier

n.
  1. A carpentry obstruction, stockade, or other obstacle made in a passage in order to stop an enemy. (Fort.)
  2. A fortress or fortified town, on the frontier of a country, commanding an avenue of approach.
  3. 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.
  4. Any obstruction; anything which hinders approach or attack.
  5. Any limit or boundary; a line of separation.
    'Twixt that [instinct] and reason, what a nice barrier!

Phrases & Compounds
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.