a method of extracting essentially pure, fresh water from polluted or salt water by forcing the water under pressure against a semipermeable membrane, which passes the pure water molecules and filters out salts and and other dissolved impurities
See reverse osmosis in American Heritage Dictionary 4
noun
A process by which a solvent such as water is purified of solutes by being forced through a semipermeable membrane through which the solvent, but not the solutes, may pass.