A type of convective transport in fluids that depends on the difference in diffusion rates of at least two density-affecting components. This phenomenon was discovered in 1960 in an oceanographic context, where the two components are heat and dissolved salts. (Double diffusion can also occur in other fluid media, such as magmas, stellar and planetary interiors, and liquid metals.) Besides different diffusivities, it is necessary to have an unstable or top-heavy distribution of one component.