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radian
noun
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
See radian in American Heritage Dictionary 4
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