the basic unit of plane angle in the SI system, equal to 57°, 17′, 44.8″ (57.29578°, the angle formed at the center of a circle by two radii cutting off an arc whose length is equal to the radius of the circle): one degree equals 0.017454 radian and 360 degrees equals 2π radians: abbrev. rad
Origin of radian; from radius
A unit of angular measure equal to the angle subtended at the center of a circle by an arc equal in length to the radius of the circle, approximately 57°17&minute;44.6&second;.
Origin of radianradi(us) + –an1.
When the length of an arc of a circle is equal to the radius of the circle, the angle subtended by that arc equals one radian.
From radi(us) + -an