Radiative transfer is the transfer of energy via electromagnetic radiation (EMR) through a gas or plasma, in particular, for a gas not so transparent that EMR merely misses all impediments. For such a less-than-transparent gas, 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.