A solid slice of space starting from a point and extending to infinity in a region of directions that can be indicated by an area on the unit sphere The size of this area is a measure of the overall solid angle, which is given in steradians
an angle formed by three or more planes intersecting at a common point (the vertex)
A portion of the whole of space about a given point, bounded by a conical surface with its vertex at that point and measured by the area cut by the bounding surface from the surface of a sphere of unit radius centered at that point See steradian