1) The separation of substances in solution by means of their unequal diffusion through semipermeable membranes; especially: such a separation of colloids from soluble substances.

2) Either of two medical procedures to remove wastes or toxins from the blood and adjust fluid and electrolyte imbalances by utilizing rates at which substances diffuse through a semipermeable membrane: a: the process of removing blood from an artery (as of a kidney patient), purifying it by dialysis, adding vital substances, and returning it to a vein--called also hemodialysis b: a procedure performed in the peritoneal cavity in which the peritoneum acts as the semipermeable membrane--called also peritoneal dialysis.