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) as a result of random motion of the molecules or atoms. Diffusion is driven by a GRADIENT in chemical potential of the diffusing species.

A gradient is the change in the value of a quantity e.g. concentration, pressure, or temperature with the change in another variable, USUALLY distance. 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.

The word diffusion derives from the Latin word, diffundere, which means "to spread way out".

A distinguishing feature of diffusion is that it depends on particle random walk, and results in mixing or mass transport without requiring directed BULK motion. Bulk motion, or bulk FLOW, is the characteristic of advection.[1] The term convection is used to describe the combination of both transport phenomena.