What does the elastic modulus measure in materials?

The elastic modulus is a fundamental property of materials that quantifies their stiffness. Specifically, it measures a material's ability to deform elastically when a force is applied; that is, how much it will stretch or compress under a certain load while returning to its original shape once the load is removed. A higher elastic modulus indicates a material that is stiffer and less prone to deformation under stress, while a lower elastic modulus suggests greater flexibility. This characteristic is crucial in engineering and materials science, as it helps inform decisions about how materials can be used in structures and applications where specific mechanical properties are required.

The other options address different material properties unrelated to the elastic modulus. For instance, thermal conductivity pertains to a material's ability to conduct heat, density relates to mass per unit volume, and surface elasticity involves how a material responds to deformations at its surface level, but none of these directly connect to the stiffness measurement that the elastic modulus provides.