For the aquarium industry; Reverse osmosis is a filtration process that uses pressure to force water through a filter that retains the impurities on one side and allows the pure water to pass to the other side. More formally, it is the process of forcing water from a region of high impurity concentration through a membrane to a region of low impurity 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 a 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 here is semi permeable, meaning it allows the passage of solvent (water) but not of solute (impurities).