I understand what diffusion of mass or diffusion of heat means. I know that the conversion of the dynamic head of the flow into static pressure in a stator or diffuser is called 'diffusion'. But in the case of diffuser what is being 'diffused'? What is 'diffusion' in a stator/diffuser?
"Diffusion" in this context means taking a flow of high-velocity gas and converting its kinetic energy into compression work, to compress that gas. In the specific case of a centrifugal compressor, as used for example in car engine turbos, an impeller forcefully slings air up to high speed and shoots it into a diffuser, which is a confined space where it the incoming air "piles up" , slows down, and gets squeezed up to higher pressure.
A Diffusor in this context is the opposite of a nozzle. While a nozzle speeds up a fluid/air flow by diminishing flow diameter, diffusion does the opposite by giving it more room, thus slowing it down and increasing pressure.
This is often done to slow down air speed below mach in order to protect a rotor/stator from damage.