Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. This movement occurs because molecules are in constant random motion, and they are more likely to move from an area where they are crowded to an area where they are less crowded.

In the case of gases, diffusion is driven by differences in partial pressure. Gases will move from areas of higher partial pressure to areas of lower partial pressure. This is how, for example, oxygen diffuses from the alveoli of the lungs into the blood, and carbon dioxide diffuses from the blood into the alveoli.