The radian (symbol rad) is the unit for measuring angles, and is the standard unit of angular measure used in many areas of mathematics.
The length of an arc of a unit circle is numerically equal to the measurement
in radians of the angle that it subtends; one radian is just under 57.3 degrees.
An arc of a circle with the same length as the radius of that circle subtends an
angle of 1 radian. The circumference subtends an angle of 2π radians.
A complete revolution is 2π radians (shown here with a circle of radius one and
thus circumference 2π).
Conversions