Anelasticity is a characteristic of viscoelastic materials that depends on some physical properties of the material itself. Anelasticity is the opposite of elasticity.
An anelastic material subjected to stress undergoes deformations that are not proportional to the stresses and that do not disappear when the force is zeroed. In other words, an anelastic material will fully recover to its original state on the removal of load.
The response of a solid to stress such that the stress deformation dependence contains, in addition to an instantaneous elastic part, a component whose equilibrium value is reached after a certain time (relaxation time) depending on the nature of the solid and on the temperature; this delayed response is due to the achievement of equilibrium by defects (impurities, dislocations, etc.) or excitations (electrons, phonons, etc.), whose energy depends on the effort.