Dialysis. Separation of substances in solution by means of their unequal diffusion through semipermeable membranes.The process of separating larger molecules from smaller molecules that occurs in kidney dialysis for example, where a machine takes a role of a kidney and filtrates blood.eparation of substances in solution by means of their unequal diffusion through semipermeable membranes.The process of separating larger molecules from smaller molecules that occurs in kidney dialysis for example, where a machine takes a role of a kidney and filtrates blood.