Reverse Osmosis is a process by which a solvent pass through a semi-permeable membrane in the direction opposite to that for normal osmosis when subjected to a hydrostatic pressure greater than the osmotic pressure.

In normal osmosis, the solvent flows from a region of low solute concentration to high solute concentration region across a semi-permeable membrane along the concentration gradient of solvent molecules. On application of an external pressure, the flow of the solvent molecules across its concentration gradient is reversed, which is known as reverse osmosis. Thus the process of reverse osmosis is against the concentration gradient and takes place with application of pressure which is greater than osmotic pressure so it is an active process and not a passive one.

(Active transport is the transport of molecules or ions across a cell membrane against its concentration gradient -from a region of lower concentration to higher concentration using energy).