Word Study
radian
CIDE DICTIONARY
radian, n. [From Radius.].
An arc of a circle which is equal to the radius, or the angle measured by such an arc. [1913 Webster]
OXFORD DICTIONARY
radian, n. Geom. a unit of angle, equal to an angle at the centre of a circle the arc of which is equal in length to the radius.
Etymology
RADIUS + -AN
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