Osmosis is the movement of water molecules from an area of high concentration across a semipermeable membrane to an area of low concentration. The semipermeable membrane permits water molecules to pass through it, but not the solute molecules. The diffusion of water molecules from the hypotonic left side into the hypertonic right side will cause the build-up of pressure on the right. This is called the osmotic pressure.

This simulation demonstrates this process. You will observe that there will be more collisions on the membrane from the molecules in the right compartment, after running the simulation for a while. This results in net pressure from the right side.

See a simulation that visualizes osmotic pressure.