Diffusion can be defined as

Diffusion, in biology terms, can be defined as a process when molecules move from a high concentration area, to an area of low concentration. To illustrate, if you were to drop a drop of red food coloring in clear water, the way the color is spreading throughout the water would be an example of diffusion.

Diffusion is the net movement of molecules or atoms from a region of high concentration to a region of low concentration

This is also referred to as the movement of a substance down a concentration gradient.