A carpentry obstruction, stockade, or other obstacle made in a passage in order to stop an enemy.
- A structure, such as a fence, built to bar passage.
- Something immaterial that obstructs or impedes: Intolerance is a barrier to understanding. See Synonyms at obstacle.
- Physiology A membrane, tissue, or mechanism that blocks the passage of certain substances.
- Ecology A physical or biological factor that limits the migration, interbreeding, or free movement of individuals or populations.
- A boundary or limit.
- Something that separates or holds apart.
- A movable gate that keeps racehorses in line before the start of a race.
- The palisades or fences enclosing the lists of a medieval tournament. Often used in the plural.
- Geology An ice barrier.