steradian

In SI, the unit of solid angle, equal to the solid angle which cuts off, on the surface of a sphere, an area equal to the area of a square whose sides are the same length as the radius of the sphere. The entire surface of a sphere subtends 4π steradians. Symbol, sr.

One steradian is approximately equal to 3,283 square degrees or 1.18 × 10⁷ square minutes or 4.25 × 10¹⁰ square seconds.