Pressure and normal stress in fluid mechanics

[asteroid21]

Hi, everyone.
I am learning fluid mechanics. One book says that for a infinitesimal fluid cell, surface force includes
1) the pressure, which is imposed by the outside fluids surrounding the concerned fluid cell
2) the shear and normal stress, which result in shear deformation and volume change, respectively.

But I think normal stress is pressure. Can anyone help me clarify?
Thank you.

 

[Chestermiller]

If the fluid is in static equilibrium, then only the pressure contributes. But, if the fluid is deforming, then there are viscous stresses in the fluid over and above the pressure; both go into determining the force per unit area. The overall stress tensor is a linear combination of the pressure (which is isotropic) and the viscous stresses, which, for an incompressible fluid, are not isotropic. The normal stress on an element of area is the sum of the pressure plus the normal viscous stress.

Chet