1. In osmosis only water or solvent molecules diffuse from their higher concentration to lower concentration, while in diffusion all type of matter can diffuse i.e. diffusion of gases, solids as well as of liquids.

2. In osmosis diffusion of solvent molecules or water molecules occur through semipermeable membrane. In simple diffusion semipermeable membrane is not required.

Thus osmosis is a special case of diffusion in which only solvent or water molecule diffuse from low to high concentration through semipermeable membrane.