Osmosis is the diffusion of water molecules across a semipermeable membrane.
Osmosis is a special kind of diffusion that applies only to water molecules.
Like diffusion, osmosis is a passive process:
Water diffuses from an area of low solute concentration (high water potential) to an area of high solute concentration (low water potential).
Low solute concentration = dilute solution = high water potential!
High solute concentration = concentrated solution = low water potential!
Zero solute concentration = pure water = zero water potential!
A semipermeable membrane only allows certain molecules to pass through. In the case of osmosis, water is able to move through, but large molecules like proteins are not.
Visking tubing is often used as a semi-permeable membrane in experiments.