Viscoelastic materials exhibit the properies of both solids and liquids, with deformation rates dependent on both the viscous stress and elastic stress rate. In a Maxwell viscoelastic material the strain rate $D$ is proportional to the sum of the stress $\tau$ and the stress rate $\dot\tau$,
This is a companion discussion topic for the original entry at https://www.underworldcode.org/articles/viscoelasticity/