An object is said to be deformed by external forces if it is stretched, squeezed or twisted. After the removal of the external force, the object may or may not restore its original shape or size.

• If the object completely restores its original shape or size after the removal of the force, it is said to be perfectly elastic object.
• If an elastic object restores its original shape or size after the removal of the deforming force, such a deformation is called elastic deformation..

Elasticity is the property of matter by virtue of which the deformed body returns to its initial configuration after the removal of the deforming force. All objects behave as perfectly elastic bodies until the deforming force reaches to a certain limit. The maximum force that the object can withstand without being permanently deformed is called the elastic limit. The nature of the substance of which the material is made, is one of the factors that determine the elastic limit of a material.

Plastic deformation is deformation where irreversible changes occur on shape or size after it is being deformed even after the removed of the deformaly. Objects that are permanently deformed by a force are called plastic objects. An object is said to be plastic if it experiences a deforming force of magnitude beyond its elastic limit.