radian

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ra·di·an (rad),

(rā'dē-ăn),
A supplementary SI unit of plane angle.
[L. radius, ray]

ra·di·an

(rad) (rā'dē-ăn)
A supplementary SI unit of plane angle.
[L. radius, ray]

radian

(rā′dē-ăn)
1. A unit of angular measurement equivalent to 57.295 degrees. It is subtended at the center of a circle by an arc the length of the radius of the circle.
2. In ophthalmometry, a lens of 1 radian would have one plane surface equal in length to the radius of curvature of the curved surface.