It is the phenomenon of water flow through a semi permeable membrane that blocks the transport of salts or other solutes through it.
Osmosis is a fundamental effect in all biological systems.
It is applied to water purification and desalination, waste material treatment, and many other chemical and biochemical laboratory and industrial processes. The flow may be stopped, or even reversed by applying external pressure on the side of higher concentration. In such a case the phenomenon is called reverse osmosis. If there are solute molecules only in one side of the system, then the pressure that stops the flow is called the osmotic pressure. The movement of a solute molecule within a solvent is over damped by the solvent molecules that surround it.