To get an understanding of the rheological principles and the behaviour of polymer materials, we can start with linear viscoelasticity.
After imposing and holding a constant deformation, an elastic solid generates a constant stress. For a Newtonian liquid, the result is a stress peak over a very short period of time.
A viscoelastic material shows an in-between behaviour: first the stress gets to a maximum after which it decreases exponentially to a constant level. For viscoelastic liquids this constant is zero. For viscoelastic solids, the constant is the equilibrium stress.