Plasticity is the opposite of elasticity. The deformation which accompanies a tensile force is called elongation or lengthening, while that accompanying a compressive force is called shortening. The deformation that accompanies bending is called deflection. It is important not to confuse stiffness and strength. They do not necessarily go together.
Many materials deform before they break.
N.B.This is a ratio and as such does not have any units. Hookes Law states that stress is proportional to strain within the elastic limit of the material. The difference between stiffness and strength should now be clearer, and the following examples may help. We have been considering mechanical forces or energy applied to materials. An analagous situation applies for heat energy as well.