A Viscoelastic fluid is a kind of non Newtonian fluid, this means it does n't follow the Newton's Viscosity Law, or at least they do n't follow it to a certain extent (it is broken beyond a small applied shear stress/strain).
This "Law" is the following equation:
Tauxy = -Mu * del(Vy)/del(x)
where:
Tauxy is the shear stress the fluid will experiment from a shear force in the "x" direction
Mu : the Dinamic Viscosity of the Fluid
Vy : The Fluid Velocity in the "y" direction
Viscoelastic fluids follow a different relation which is non-linar between the "velocity deformation" and strain