The diffusion coefficient is a measure of how quickly particles (atoms, or ions) move through a medium due to random thermal motion. It quantifies the rate at which particles spread out from areas of high concentration to areas of low concentration. It is important to realize that this chemical diffusion differs from the tracer or Einstein diffusion, which is due to thermal fluctuation. Only a low concentrations of the diffusing particles, the chemical and the tracer diffusion coefficients are the same.