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.