Property of a material which is viscous but which also exhibits certain elastic properties such as the ability to store energy of deformation, and in which the application of a stress gives rise to a strain (or vice versa) that approaches its equilibrium value over a significant time interval. Viscoelastic materials can’t be accurately described using just classical fluid mechanics (Newtonian) or classical elastic mechanics (Hookean). Rheology (flow behavior) of polymer melts is an example of viscoelasticity, which is related to the structure, length and interaction of their molecules.