The force per unit area required to sustain a constant rate of fluid movement. Mathematically, shear stress can be defined as:

τ = F/A,

where
τ = shear stess F = shear force A = Area acted on by the shear force.

If a fluid is placed between two parallel plates spaced 1.0 cm apart, and a force of 1.0 dyne is applied to each square centimeter of the surface of the upper plate to keep it in motion, the shear stress in the fluid is 1 dyne/cm^{2} at any point between the two plates.