Someone or something that excludes; a person or device that prevents access or prevents something from entering or being used.
From 'exclude' + '-er' suffix for agent noun (one who does). The '-er' suffix creates a noun from the verb 'exclude.'
In beekeeping, a 'queen excluder' is a screen that keeps the large queen bee out of certain hive areas so workers can lay their own eggs—it's a simple barrier with profound effects on bee society.
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