In material properties, the term 'elasticity' refers to:

Elasticity refers specifically to the ability of a material to return to its original shape after the removal of a load or force. This is a fundamental property of materials that defines how they deform under stress and then recover once that stress is eliminated. The elastic behavior is typically described by Hooke's Law, which states that the deformation (strain) of a material is directly proportional to the applied stress, up to a certain limit known as the elastic limit.

When a material experiences elastic deformation, it does not undergo any permanent change in shape or size; once the forces are removed, it reverts back to its initial configuration. This characteristic is crucial in many structural applications, where materials are subjected to loads and need to perform under those loads without undergoing permanent deformation.

In contrast, the other options discuss different material properties. The ability to withstand shear stress is related to shear strength; thermal expansion involves how a material expands or contracts with changes in temperature; and density reflects the mass per unit volume of a material, which does not directly relate to its elastic behavior.