Etymology
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radian (n.)

"angle subtended at the center of a circle by an arc equal in length to the radius," 1879, from radius.

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Definitions of radian

radian (n.)
the unit of plane angle adopted under the Systeme International d'Unites; equal to the angle at the center of a circle subtended by an arc equal in length to the radius (approximately 57.295 degrees);
Synonyms: rad
From wordnet.princeton.edu