Steradian ($\mathrm{sr}$)

A steradian is the solid angle subtended at a sphere's center by a patch of surface area $R^2$. The whole sphere subtends $4\pi \mathrm{sr}$. In the SI, the steradian has unit one; retaining its symbol identifies a solid angle, just as radians identify an angle. A dimensionless quantity still needs its physical meaning specified.^[ref]