Osmosis

Osmosis, diffusion of a solvent through a semipermeable membrane that separates 2 solutions of different concentration. The movement is from the more dilute to the more concentrated solution, because of the thermodynamic tendency to equalize the concentrations. The liquid flow may be opposed by applying pressure to the more concentrated solution; the pressure required to reduce the flow to zero from a pure solvent to a given solution is known as the osmotic pressure of the solution.