What is the unit for a solid angle?

The unit for a solid angle is the steradian (sr). A solid angle is a three-dimensional analogue of a two-dimensional angle and measures the size of an angle in three-dimensional space. One steradian is defined as the angle subtended at the center of a sphere by an area on the surface of the sphere equal to the square of its radius.

In contrast to the steradian, the radian is specifically used for measuring angles in two dimensions, while degrees and revolutions are also units for measuring angles but are not applicable to solid angles. Degrees measure the arc-length of a circular segment, and a revolution quantifies the complete turn around a point, both in a planar context rather than in three-dimensional space. Thus, the steradian is the correct and standard unit for quantifying solid angles.