The tendency in fluids to mix, or become equably diffused, when in contact. It was first observed between fluids of differing densities, and as taking place through a membrane or an intervening porous structure. An older term for the phenomenon was Osmose.
the movement of a solvent (like water) from an area of lesser to greater concentrations so that the solvent will be evenly distributed. So for example, if you put a cell in pure water (that has no dissolved chemicals in it), the water outside would move inside to increase the concentration of water inside the cell. Because most of the dissolved material in a cell cannot get out (to lower the concentration of water outside the cell), water will keep coming inside until it bursts the cell. This movement of water is a special case of the principle of diffusion, which applies to dissolved substances, which also tend to distribute evenly if there are no barriers