radian

[rey-dee-uh n] /ˈreɪ di ən/
noun, Mathematics
1.
the measure of a central angle subtending an arc equal in length to the radius: equal to 57.2958°.
Abbreviation: rad.
Origin
1875-80; radi(us) + -an
British Dictionary definitions for radian

radian

/ˈreɪdɪən/
noun
1.
an SI unit of plane angle; the angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius. 1 radian is equivalent to 57.296 degrees and π/2 radians equals a right angle rad
Word Origin
C19: from radius
Word Origin and History for radian
n.

"angle subtended at the center of a circle by an arc equal in length to the radius," 1879, from radius.

radian in Medicine

radian ra·di·an (rā'dē-ən)
n.
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.

radian in Science
radian
  (rā'dē-ən)   
A supplementary unit of the International System used in angular measure. One radian is 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°17'44.6".