barrier

(bărˈē-ər)

1. A structure, such as a fence, built to bar passage.
2. Something immaterial that obstructs or impedes: Intolerance is a barrier to understanding. See Synonyms at obstacle.
3. Physiology A membrane, tissue, or mechanism that blocks the passage of certain substances.
4. Ecology A physical or biological factor that limits the migration, interbreeding, or free movement of individuals or populations.
5. A boundary or limit.
6. Something that separates or holds apart.
7. A movable gate that keeps racehorses in line before the start of a race.
8. The palisades or fences enclosing the lists of a medieval tournament. Often used in the plural.
9. Geology An ice barrier.