What does DIFF stand for?

Diffusion

Movement of atoms in solids. Heat provides the energy for atom movement. The gradual mixing of the molecules of 2 or more substances by random molecular motion. Diffusion is a process that happens when a substance moves from an area of higher concentration to an area of lower concentration.  Diffusion is the process whereby one substance moves through another. The substances become mixed without us needing to stir them together.