The steradian (symbol: sr) or square radian is an SI unit. It is like a radian, but for smooth 3D objects, for example, a sphere. A steradian is about one twelfth the surface area of a sphere, regardless of how big the sphere is.[1]
A steradian on a sphere
↑Weisstein, Eric W. "Steradian". mathworld.wolfram.com. Retrieved 2020-03-27.