Rosseland mean opacity
= Rosseland mean opacity
{c}
{title2=$\kappa_R$}
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.