Radian

**Semantic type:** Quantitative Concept

**Definition:** A SI-derived unit of plane angle measurement defined and named by James Thomson in 1873. One radian is the plane angle between two radii of a circle that cuts off on the circumference an arc equal in length to the radius. There are 2pi radians in a complete circle. SI units: m/m = 1.

**Synonyms:** - Rad - rad - rad - rad - rad - rad - radian - radian