When two solutions of unequal concentrations are separated by a semipermeable membrane, flow of solvent takes place from dilute to concentrated sides, due to osmosis.

If, however, a hydrostatic pressure in excess of osmotic pressure is applied on the concentrated side, the solvent flow reverses, i.e. solvent is forced to move from concentrated side to dilute side across the membrane.

This is the principle of reverse osmosis.

Thus, in reverse osmosis methods, pure solvent (water) is separated from its contaminates, rather than removing contaminants from the water.

This membrane filtration is sometimes also called "Super-filtration" or "Hyper filtration".