The textbook definition of diffusion is the movement of molecules or particles from an area of higher concentration to an area of lower concentration, continuing until equilibrium is reached. The defining quality of diffusion is that it takes place because the diffusing substance exists in a higher concentration in one place and a lower concentration in another. If the substance moves for a different reason, such as pressure or force, that’s not diffusion. To get scientific with it, diffusion means a passive movement of molecules down a concentration gradient.
Here are several real-world, practical examples of diffusion from everyday life. That’s what science is!