The
radian is the standard unit of angular measure, used in many areas of
mathematics. An
angle's measurement in radians is numerically equal to the length of a corresponding arc of a
unit circle; one radian is just under 57.3
degrees (when the arc length is equal to the radius). The unit was formerly an
SI supplementary unit, but this category was abolished in 1995 and the radian is now considered an
SI derived unit.