Osmosis is a spec

Osmosis is a special kind of diffusion, in which water diffuses across the cell membrane. The rate and direction of osmosis depends upon both pressure and concentration gradient.

In osmosis, the flow of solvent molecules is along their concentration gradient i.e. from more dilute to less dilute solution. Reverse osmosis occurs when pressure applied does not allow this osmosis to take place and instead moves from the less dilute to the more dilute solute basis the pressure applied.