Radiative diffusion

In an optically thick medium whose radiation field is nearly isotropic and thermal, the first angular correction to the Planck function produces the diffusive fluxwhere ${\kappa }_R$ is the Rosseland mean opacity.

The Rosseland mean opacity is the harmonic frequency average weighted by $\partial B_{\nu }/\partial T$. It controls radiative diffusion because flux preferentially travels through low-opacity frequencies.

It is a frequency-averaged opacity appropriate to the radiative-diffusion limit, not the general opacity concept itself.