Diffusion can be defined as

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.

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.