Radiative transfer is the transfer of energy via electromagnetic radiation (EMR) through a gas or plasma, in particular, if the gas not so transparent that EMR merely passes through it. In that case, absorption, emission, and scattering of the constituent atoms are basic to the process.
Radiative transfer is characterized by the quantity radiance, the rate of energy flowing at a given time through a given area at a given location. Spectral radiance, in turn, is the radiance at a given wavelength.
An equation of radiative transfer is an equation that describes this process, relating the transmission of energy to quantities characterizing the likelihood of absorption, emission, and scattering.