Q.What do you mean by deformation?

Every solid consists of atoms held in a definite equilibrium arrangement by the mutual forces between them. When a deforming force displaces these atoms slightly, internal restoring forces are set up that try to bring the body back to its original shape and size — but whether, and how completely, a solid actually recovers depends on the material.

A body is called elastic, and the property is called elasticity, if it regains its original shape and size once a deforming force is removed; the recovery is possible because the internal restoring forces are strong enough to pull the displaced atoms all the way back. Metals, rubber and quartz are common elastic materials, with quartz fibre regarded as the closest real approximation to a perfectly elastic solid (one that recovers completely and instantaneously).

A body is called plastic, and the property plasticity, if it does not regain its original shape or size and instead retains the altered form even after the deforming force is removed — here the restoring forces are too weak to pull the atoms back. Clay, putty and thick mud are typical plastic materials, with putty the closest real approximation to a perfectly plastic body. No real material is perfectly elastic or perfectly plastic; every material's actual behaviour lies somewhere on the spectrum between these two idealised extremes, and even a single material can behave nearly elastically for small deforming forces while becoming increasingly plastic as the applied force grows larger (see the full stress-strain curve, which traces exactly this transition).