Definition:
In geometry and radiometry, a solid angle is a measure of the amount of the field of view from some particular point that a given object covers.

For an arbitrary surface that subtends a region at a point (the vertex), the solid angle $\Omega$ is defined as the ratio of the area $A$ projected onto the surface of a sphere to the square of the sphere's radius $r$: Source:https://commons.wikimedia.org/wiki/File:Angle_solide_coordonnees.svg

$$\Omega = \frac{A}{r^2}$$ where:

• $\Omega$ is the solid angle,
• $A$ is the area of the projection on the surface of a sphere of radius $r$,
• $r$ is the radius of the sphere centered at the vertex.

Unit
The SI unit of solid angle is the steradian ($sr$).

Notes:

• A full sphere subtends a solid angle of $4\pi$ steradians.
• Solid angle is the three-dimensional analogue of the planar angle (measured in radians).
• It is widely used in fields such as radiometry, photometry, computer graphics, and optical engineering.