Definition of radian noun from the Oxford Advanced Learner's Dictionary

 

radian

 noun
noun
BrE BrE//ˈreɪdiən//
 
; NAmE NAmE//ˈreɪdiən//
 
(geometry)
 
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a unit used to measure an angle, equal to the angle at the centre of a circle whose arc is the same length as the circle’s radius Word Origin late 19th cent.: from radius + -an.
See the Oxford Advanced American Dictionary entry: radian