Steradian

The steradian (ste from Greek stereos, solid) is the SI supplementary unit of solid angle. The symbol is sr.

It is defined as "the solid angle subtended at the center of a sphere of radius r by a portion of the surface of the sphere having an area r²". It is also equal to the spherical area of a polygon having an angle excess of 1 radian.

There are 4π steradians in a complete sphere.