noun Mathematics.

- the measure of a central angle subtending an arc equal in length to the radius: equal to 57.2958°. Abbreviation: rad

noun

- an SI unit of plane angle; the angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius. 1 radian is equivalent to 57.296 degrees and π/2 radians equals a right angleSymbol: rad

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

n.

- A unit of angular measure equal to the angle subtended at the center of a circle by an arc equal in length to the radius of the circle.

- A supplementary unit of the International System used in angular measure. One radian is equal to the angle subtended at the center of a circle by an arc equal in length to the radius of the circle, approximately 57°17′44.6″.