Diffusion is the net movement of molecules or atoms from a region of high concentration (or high chemical potential) to a region of low concentration (or low chemical potential). This is also referred to as the movement of a substance down a
concentration gradient. A
gradient is the change in the value of a quantity (e.g., concentration,
pressure,
temperature) with the change in another variable (usually
distance). For example, a change in concentration over a distance is called a concentration gradient, a change in pressure over a distance is called a
pressure gradient, and a change in temperature over a distance is a called a
temperature gradient.