Official permissions to proceed or act, or the space or distance between two objects.
From 'clear' (Old French 'cler', from Latin 'clarus'). The word evolved from 'making clear' to 'making clear to pass' to 'official permission to pass.'
Airport security clearances and the clearance between a truck and a tunnel roof both use the same word because they're both about confirming something is 'clear' to pass through safely.
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