A three-dimensional angle.

The ratio of the area on the surface of a sphere to the square of the radius of that sphere. It is expressed in steradians (sr).

an angle formed by three or more planes intersecting at a common point (the vertex)

a three dimensional one that radiates from the apex of a sphere

steradian

A measure of that portion of space about a point bounded by a conic surface whose vertex is at the point. It is defined as the ratio of intercepted surface area of a sphere centered on that point to the square of the sphere's radius. It is expressed in steradians.

The solid angle, Ω, that an object subtends at a point is a measure of how big that object appears to an observer at that point. For instance, a small object nearby could subtend the same solid angle as a large object far away. The solid angle is proportional to the surface area, S, of a projection of that object onto a sphere centered at that point, divided by the square of the sphere's radius, R.