There are two supplementary SI units that are used to distinguish between quantities which are of a different nature. Otherwise, they would have the same SI units. For example, angular velocity is specified in units of radians per second (rads^{1}). Since radian is dimensionless, without the supplementary unit the angular velocity would have specified as s^{1}, which is the same as that of frequency.
The supplementary units are shown below:
Supplementary quantity 
Name 
Symbol 
plane angle 
radian 
rad 
solid angle 
steradian 
sr 
Definition
Radian  It is the plane angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius.
Steradian  It is the solid angle that, having its vertex in the center of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.
