Radian

radian

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 angle

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radian [′rād·ē·ən]

(mathematics)

The central angle of a circle determined by two radii and an arc joining them, all of the same length.

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Radian 

the central angle subtended in a circle by an arc whose length is equal to the circle’s radius. A radian is approximately equal to 57°17′44.8″. The radian is used as the unit of measurement of angles in what is known as circular, or radian, measure. If the circular measure of an angle is a radians, the angle contains (180a/π)°. Conversely, if the degree measure of an angle is n°, the circular measure is πn/180 radians. For example, angles of 30°, 45°, 60°, 90°, and 180° correspond to angles of π/6, π/4, π/3, π/2, and π radians, respectively.