Reverse osmosis is a filtration process that is often used for desalinating water. In order to produce fresh water from seawater, it works by using pressure to force seawater through a membrane, retaining the salt and other contaminants on one side and allowing the pure water to pass to the other side. 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.