A diagram of diffusion happening. The first diagram shows particles in a liquid. The second shows the same liquid a few seconds later after the particles have spread out

Diffusion is a process where molecules of a material move from an area of high concentration (where there are many molecules) to an area of low concentration (where there are fewer molecules)until it has reached equilibrium (molecules evenly spread).

Diffusion usually happens in a mixture in gas, a liquid and occasionally colloids. It is possible to see diffusion happening when two liquids are mixed in a transparent container. It describes the constant movement of particles in all liquids, gases and colloids. These particles move in all directions bumping into each other.