The opacity of a gas of given composition, temperature, and density, averaged over the various wavelengths of the radiation being absorbed and scattered. The radiation is assumed to be in thermal equilibrium with the gas, and hence to have a black-body spectrum. The Rosseland mean opacity is useful for calculating the total amount of energy absorbed over all wavelengths, which is required in calculations of stellar structure. It is named after the Norwegian astrophysicist Svein Rosseland (1894–1985).