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English definition of “radian”

radian

noun [C]  /ˈreɪ·di·ən/ us  
geometry the measure of an angle at the center of a circle opposite an arc (= part of the edge of a circle) that is the same length as the radius (= line from the center to the edge of the circle)
(Definition of radian from the Cambridge Academic Content Dictionary © Cambridge University Press)
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