Definition:
If an object of arbitrary shape subtends a region at the center of a sphere, and its projected area on a spherical surface of radius r is A, then the solid angle (denoted by the symbol $\Omega$) subtended by the object at the center of the sphere is:$$\Omega = \frac{A}{r^2}$$ Unit: steradian (sr)
(Image source: https://commons.wikimedia.org/wiki/File:Angle_solide_coordonnees.svg)