Primary
Radian (rad) ○◂｜Definition｜1st｜20251119205401-00-⌔
Radian
The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. It is defined such that one radian is the angle subtended at the center of a plane circle by an arc that is equal in length to the radius.1 The unit is defined in the SI as the coherent unit for plane angle, as well as for phase angle.2 Angles without explicitly specified units are generally assumed to be measured in radians, especially in mathematical writing.3
Printed 2026-06-28.
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Link to original Footnotes
The International System of Units (PDF), V4.01 (9th ed.), International Bureau of Weights and Measures, Jun 2026, p. 138, ISBN 978-92-822-2272-0: “One radian is the angle subtended at the centre of a circle by an arc that is equal in length to the radius.” ↩
The International System of Units (PDF), V4.01 (9th ed.), International Bureau of Weights and Measures, Jun 2026, p. 138, ISBN 978-92-822-2272-0: “The radian is the coherent unit for plane angle… The radian is also the coherent unit for phase angle.” ↩
Ocean Optics Protocols for Satellite Ocean Color Sensor Validation, Revision 3. National Aeronautics and Space Administration, Goddard Space Flight Center. 2002. p. 12. ↩
Secondary
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