| n. | 1. | (Math.) An arc of a circle which is equal to the radius, or the angle measured by such an arc. |
| Noun | 1. | radian - 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) Synonyms: rad |