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 *r*^{2}". It is also equal to the spherical area of a polygon having an angle excess of 1 radian.

Basically this is the 3-dimensional version of the radian.

There are 4&pi **steradians** in a complete sphere.