Barrier

A barrier is an obstacle that restricts movement, flow, or interaction between two areas or states. Barriers can be tangible, such as walls or rivers, or conceptual, such as laws or social norms. They may serve to protect, delineate, or regulate access and exchange.

Physical barriers include walls, fences, gates, dams, and road blocks. Natural barriers include mountains, deserts, oceans, and dense vegetation, which can limit dispersal of people, organisms, or goods. Geographic barriers influence patterns of migration, trade, and biodiversity.

Biology uses barriers at multiple scales: skin and mucous membranes protect against pathogens; cell membranes regulate substance passage; the blood–brain barrier restricts most chemicals from entering the brain. Chemical barriers include pH, salinity, and surface tension that limit diffusion in ecosystems or industrial processes.

In human systems, barriers arise from policy and economics, such as borders, tariffs, licenses, and standards that impede entry or transfer of goods or people. Information and digital barriers include firewalls and access controls that limit data flow.

Barriers are designed, evaluated, and sometimes removed as conditions change. They can protect safety, manage resources, or influence strategic outcomes, but may also create costs or hinder beneficial exchange.