1. A material formation or structure, such as a mountain range or wall, that prevents passage or access.

2. Something immaterial that obstructs or impedes: Lack of education can be a barrier to success.

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 movable gate that keeps racehorses in line before the start of a race.

6. often barriers The palisades or fences enclosing the lists of a medieval tournament.

7. Geology An ice barrier.