radians in one complete revolution of 
and 1 radian 
[M1.6, M2.1]
is: a dimensionless derived unit of plane angle. [M1.6, M2.1]
is defined: by requiring that, for any circle, the arc subtended at the circumference by an angle of 1 radian at the centre has a length equal to that of the circle's radius; so there are radians in one complete revolution of and 1 radian [M1.6, M2.1]
See degree.
Copyright 1997, The Open University