barrier
bar·ri·er
 /ˈberijɚ/  noun  ,  pl   -ers [  count  ]
  1 : something (such as a fence or natural obstacle) that prevents or blocks movement from one place to another
   Concrete barriers surround the race track to protect spectators.
   The tree's roots serve/act as a barrier against soil erosion.
   The mountain range forms a natural barrier between the two countries.
   barrier beaches
  — see also barrier island, barrier reef, crash barrier
  2 a : a law, rule, problem, etc., that makes something difficult or impossible
   Both leaders are in favor of removing trade barriers.
   Cultural/social barriers have made it hard for women to enter many professions.
   He argues that regulations should not be viewed as barriers to progress.
  2 b : something that makes it difficult for people to understand each other
  — often + between
   The lecture was about finding ways to break through barriers between the social classes.
   Age can be a big barrier between parents and children.
   A language barrier existed between the two countries. [=people in the two countries did not understand each other because they spoke different languages]
  3 : a level, amount, or number that is difficult to get past
   His fastball broke the 100-mph barrier. [=he threw a baseball over 100 mph]
   a price barrier [=a price that is regarded as a limit]
  — see also sound barrier