A barrier is something that blocks or stops the progress of something or someone. It could be a physical object, like a wall or fence, that prevents movement, or it can be a figurative obstacle that hinders progress or communication. Full definition
We conclude that near future ocean acidification levels is not an immediate threat to the early life history stages of Great Barrier Reef corals in the foreseeable future. (coralcoe.org.au)
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This seems like a business with low barriers of entry and where returns depend highly on management. (longtermvalue.wordpress.com)