Ability of a material to return to its original shape when load causing deformation is removed.
Elasticity is the ability to store and release energy with a spring-like sample response as described by Hook’s law of elasticity (e.g. perfectly rigid solids and springs behave as ‘Hookean’ solids).
If the strain increases linearly with increasing applied stress, the material is purely elastic. Some materials, notably polymers (plastics), exhibit a strong visco-elastic behavior, where the stress-strain relation is not linear and the sample response is strongly dependent on time and rate of load application.