I'd be the first to agree that all this can be confusing Dreddnaught. Let me try to explain (if the alcohol in my bloodstream will allow!)
Diffusion may be defined as the movement of molecules from a region of high concentration to one of lower concentration due to random molecular motion.
Diffusion of gases in other gases is most rapid and is indeed the most commonly cited form of diffusion. However, diffusion of solutes in solvents can also occur, and solids are capable of diffusing into each other, although at extremely slower rates than gases.
The rate of diffusion of molecules from any region in a substance is proportional to the concentration of the molecules in that region. In turn, the rate of diffusion into that region is proportional to the concentration of molecules in the surrounding area. What this means in practice is that the net flow is from regions of higher concentrations to regions of lower concentrations.
When a soluble salt, such as copper sulphate is dissolved in water, it may be considered to have diffused into that medium. Water evaporates into the air, which is another form of diffusion. Some types of polymers will migrate or diffuse into one another when kept in close proximity. There are even some elements that diffuse into each other under the right conditions.
Osmosis is solutions is essentially the retention of a solute behind a membrane which the solvent has passed through.