A substance that displays behavior that is both viscous and elastic is said to be viscoelastic. In this sense, viscoelastic materials are said to be a combination of the ideal (elastic) Hookean solid and the (viscous) Newtonian liquid, along with a time a dependence.
A Hookean solid is one that displays perfectly elastic behavior. This corresponds to the fact that an applied shear stress produces a shear strain in response. Recall that the shear stress (<math>\sigma</math>) is given by the applied force over the area, namely <math>\sigma = F/A</math>, and the shear strain (<math>e</math>) is given by <math>e = \Delta x/y</math>. See Figure 1 for further clarification.
[[Image:Cube.jpg|thumb|300px|Figure 1: Definitions for a shearing material.