## radian

ra·di·an 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 2? radians: abbrev.

*rad*Origin of radian

; from radius## radian

noun

*Abbr.*

**rad**

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 radian

**radi(us)**+

**–an**

^{1}.

**radian**

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.

## radian

Noun

(*plural* radians)

Origin

From radi(us) + -an