Osmosis is a special case of?

Osmosis is a special case of diffusion. 

Diffusion is the movement of particles from areas of high concentration to areas of low concentration.

Osmosis is the diffusion of water molecules through a semipermeable membrane. Osmosis only occurs when the concentrations on either side of the membrane are different. Osmosis will occur until the concentrations equalize. 

It is important to remember that osmosis considers the movement of water. Water is a solvent but we usually talk about concentration in terms of the solute. Water will move from areas of high water concentration (low solute concentration) to areas of low water concentration (high solute concentration). 

Thus, water (the solvent) will move from low salt (solute) concentrations to high salt concentrations. High solute concentrations indicate a low solvent concentration, and vice versa.