I understand mass diffusion. But what is the meaning of 'diffusion' which converts the kinetic energy to rise in pressure in a stator/diffuser?
The word "diffuse" is used in different contexts in both engineering and in the vernacular. A general definition would be similar to, as a verb, "spread out over a larger area, space, or number of participants or carriers."
With momentum diffusion, a more concentrated area of fluid momentum is spread out to a larger flow volume and to more fluid particles that have their mean velocity increased.
With heat diffusion, a concentrated area of molecular motion is likewise spread out, with larger numbers of molecules assuming more vibrational energy.
With mass diffusion, a concentration of molecular species among other species is spread out to a larger or more distant region, reducing the concentrations of the named species.
With a fluid diffuser, a concentrated area of kinetic energy of fluid motion gets directed to a region where the kinetic energy is converted (diffuses) to a higher static pressure. With subsonic flow, this requires an increase in flow area, but with supersonic flow, this requires a drop in flow area.