Definition of a radian

A radian is a unit of angular measure. A revolution of a circle has an angle of 2π radians.

A radian is a unit of measuring angles. It is shown by the symbol "rad" or, less often, ^c (for circular measure). The radian was once an SI supplementary unit, but was changed to a derived unit in 1995. A radians arc length is equal to the radii from the circle it is part of.