Which term describes a material's resistance to deformation?

Stiffness is the correct term that describes a material's resistance to deformation. It specifically refers to the degree to which a material can resist changes in shape or size when subjected to an external force. Stiffness is a critical property in engineering and materials science because it influences how structures and components perform under loading. A stiffer material will deform less compared to a less stiff material when the same load is applied.

For example, in applications such as beams, supports, and mechanical components, engineers often select materials based on their stiffness to ensure that structures can bear loads without excessive bending or stretching.

In contrast, elasticity refers to a material's ability to return to its original shape after the removal of a load. Tensile strength measures how much force a material can withstand while being stretched or pulled before failing. Hardness indicates a material's resistance to localized plastic deformation (like indentation). While each of these properties is important, stiffness directly captures the concept of resistance to deformation under applied forces.