What does "barrier" mean?

Additional senses

1. 2.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.
2. 3.An any obstruction; anything which hinders approach or attack. "Constitutional barriers." Hopkinson.
3. 4.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.]