1. Noun. 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).

Generic synonyms: Angular Unit

1. n. An arc of a circle which is equal to the radius, or the angle measured by such an arc.

1. Noun. (geometry) In the International System of Units, the derived unit of plane angular measure of angle equal to the angle subtended at the centre of a circle by an arc of its circumference equal in length to the radius of the circle. Symbol: rad ¹

¹ Source: wiktionary.com

1. a unit of angular measure [n -S]

1. An arc of a circle which is equal to the radius, or the angle measured by such an arc. Origin: From Radius. Source: Websters Dictionary (01 Mar 1998)

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Below you will find example usage of this term as found in modern and/or classical literature:

1. Plane and Spherical Trigonometry by George Neander Bauer, William Ellsworth Brooke (1917)
"Circular or radian measure. A radian is an angle of such magnitude that, ... Thus, if the arc XP is equal to the radius OX, the angle ХОР is a radian, ..."

2. Plane and Spherical Trigonometry by Leonard Magruder Passano (1918)
"CHAPTER V THE CIRCULAR OR radian MEASURE OF AN ANGLE. INVERSE TRIGONOMETRIC FUNCTIONS 47. Circular or radian Measure of an Angle. Any convenient unit may be ..."

3. Plane Trigonometry, for Colleges and Secondary Schools by Daniel Alexander Murray (1899)
"(1) 1 2 >u 7T B Since all right angles are equal, and since each radian is a ... Ex. With a protractor lay off an angle approximately equal to a radian. ..."

4. A Course in Electrical Engineering by Chester Laurens Dawes (1922)
"APPENDIX A Circular Measure—The radian The radian is circular angle ... The circle has a radius of r units and •*' the radian is subtended by an arc whose ..."

5. Elements of Plane and Spherical Trigonometry by David Andrew Rothrock (1910)
"In this system the unit of measure is the radian, the angle subtended at the center of a circle by an ... 29, the arc BC = ОС, hence the angle COB=I radian. ..."

6. A First Course in the Differential and Integral Calculus by William Fogg Osgood (1922)
"radian Measure. In Trigonometry, the radian measure of an angle was ... Then the radian measure, 6, of the angle is defined as the ratio s/r: ^S' V \9 (1) ..."

7. Trigonometry by Alfred Monroe Kenyon, Louis Ingold (1913)
"radian Measure of Angles.* A radian is a positive angle such that when its vertex ... Use of radian Measure. It is shown in geometry that two angles at the ..."

8. Mensuration by William Shaffer Hall (1893)
"Another unit angle that is often used is called a " radian." A radian is the angle at the centre of a circle subtended by an arc equal to the radius. ..."