Osmosis is the process of molecules or ions moving from an area of low concentration to an area of high concentration (they move against their concentration gradient). The process does not require energy.
This is a special case, because diffusion against a concentration gradient generally requires energy (in the form of ATP) to move molecules.
Without energy provided, diffusion is usually referred to as 'passive' and is the process of ions or molecules moving from an area of high concentration to low concentration.
Osmosis is the movement of molecules/ions from an area of low to an area of high concentration, yet it is an energy independent process.
This makes it unique or special as a form of diffusion.