A three-dimensional angle formed by a conical surface (see cone).
Considering its vertex to lie at the center
of a sphere, then a measure of its magnitude
may be obtained from the ratio between the area (L2)
of the surface of the sphere cut off by the angle, and the square (R2)
of the sphere's radius. Solid angles are measured in steradians
(sr), an angle of one steradian subtending an angle of area R2
at a distance R.
See also angle.