(Lat. diffundō, to pour out), a spreading or flowing of a liquid or a fluid, as of water, air, light: a scattering or dispersing.

Diffusion of Gases, a term in chemistry, denoting the mixing, in any relative proportions, of two gaseous bodies which do not act chemically on each other; so that whatever be their relative densities they remain permanently blended. Hence Dalton has represented the gaseous bodies as acting as cœcum to each other.