Movement of a substance from an area of higher concentration to one where it is less concentrated in

Diffusion is the movement of solutes from a higher concentration to a lower concentration.

It is important not to confuse this with osmosis, which refers to the movement of water only (from a lower concentration solution to a higher concentration solution.)

Diffusion and osmosis almost always goes hand-in-hand. A good way to remember this is if you imagine mixing a powdered juice drink. Diffusion talks about the powder's movement while osmosis talks about the water's movement.