flow and the area of the flat plate), is the local gradient and is the viscosity. The units of shear stress are Force/Unit Area. This is in M.K.S system. This can also be interpreted as . However, these are also the units of momentum flux. This is the precise reason why shear stress in a fluid can also be interpreted as the flux of momentum. The diffusion of momentum is in the direction of decreasing velocity. This means that momentum is being transferred from the fluid in the upper layers (which has greater momentum) towards the fluid that is close to the wall (which has lesser momentum due to its lower velocity). The phrase "momentum diffusion" can also refer to the diffusion of the probability for a single particle to have a particular momentum. In this case, it is the probability distribution function that diffuses in momentum space, rather than the (conserved) quantity of momentum that diffuses among many particles. References Diffusion Fluid dynamics