The unit of solid angle is the steradian (sr), which is a dimensionless unit in the International System of Units (SI) . One steradian corresponds to one unit of area on the unit sphere surrounding the apex, so an object that blocks all rays from the apex would cover a number of steradians equal to the total surface area of the unit sphere. The solid angle can also be measured in squares of angular measures such as degrees, minutes, and seconds, but the steradian is the standard unit of solid angle in the SI system.

In summary, the unit of solid angle is the steradian (sr).