Reverse Osmosis (commonly abbreviated as RO) is a water purification technology that uses a semipermeable membrane to remove ions, molecules, and larger particles from water.
In reverse osmosis, an applied pressure is used to overcome osmotic pressure driven by chemical potential differences of the solvent. Reverse osmosis can remove many types of dissolved and suspended species from water.
The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side.
To be "selective", this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as solvent molecules) to pass freely.