Reverse osmosis (RO) is the process of pushing a solution through a filter that traps the solute on one side and allows the pure solvent (permeate) to be obtained from the other side.

More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure.

This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied.

The membrane is semipermeable, meaning it allows the passage of solvent but not of solute.