What is "self-diffusion"

According to IUPAC definition, self-diffusion coefficient is the diffusion coefficient D of species i when the chemical potential gradient equals zero. It is linked to the diffusion coefficient D by the equation:

$D_i^*=D_i\frac{\partial\ln c_i}{\partial\ln a_i}.$

Here, a is the activity of the species i in the solution and c is the concentration of i. This term is commonly assumed to be equal to the tracer diffusion determined by watching the movement of an isotope in the material of interest.