What is the solid angle $dOmega$ in radiative transfer?

Question

The Wikipedia article for radiative transfer gives the following definition:

In terms of the spectral radiance, $I_{nu }$, the energy flowing across an area element of area $da$, located at $mathbf{r}$ in time $dt$, in the solid angle $dOmega$ about the direction ${hat {mathbf {n} }}$ in the frequency interval $nu +dnu$, is
$$dE_{nu }=I_{nu }(mathbf {r} ,{hat {mathbf {n} }},t)cos theta  dnu ,da,dOmega ,dt,$$
where $theta$  is the angle that the unit direction vector ${hat {mathbf {n} }}$ makes with a normal to the area element.

It isn't clear to me what the solid angle $dOmega$ is supposed to be. I would greatly appreciate it if people would please take the time to explain this.
Related: https://en.wikipedia.org/wiki/Linear_transport_theory

CGS · Accepted Answer

A solid angle is a 3 dimensional angle.  See this link and this image with red box:

Thomas Fritsch · Answer

The concept of solid angle is not special to your use-case,
but used much more widely.
The solid angle $Omega$ is simply defined as the ratio of a surface area
element $A$ of a sphere and its squared radius $R^2$:
$$Omega=frac{A}{R^2}$$

(image from this math question)
The same goes when you have a small infinitesimal solid angle $dOmega$
and surface area element $dA$:
$$dOmega=frac{dA}{R^2}$$

What is the solid angle $dOmega$ in radiative transfer?

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