Etymology
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clearance (n.)

1560s, "action of making clear," from clear (v.) + -ance. Meaning "a clear space" is from 1788. Meaning "approval, permission" (especially to land or take off an aircraft) is from 1944, American English; national security sense recorded from 1948. Clearance sale attested by 1843.

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Definitions of clearance

clearance (n.)
the distance by which one thing clears another; the space between them;
clearance (n.)
vertical space available to allow easy passage under something;
Synonyms: headroom / headway
clearance (n.)
permission to proceed;
the plane was given clearance to land
From wordnet.princeton.edu