What property describes a material's ability to deform under stress?

The property that describes a material's ability to deform under stress is ductility. Ductility refers specifically to a material's capacity to undergo significant plastic deformation before rupture or fracture. This means that a ductile material can be stretched into a wire or deformed significantly without breaking when subjected to tensile stress.

In contrast, brittleness is characterized by a material that breaks or fractures with little deformation, demonstrating minimal ability to absorb energy before failing. Toughness, on the other hand, measures a material's ability to absorb energy and plastically deform without fracturing, but it does not directly address deformability under stress. Elasticity describes how a material can return to its original shape after the stress is removed, but this does not imply the material can deform significantly; rather, it pertains to recovering from deformation. Therefore, ductility is the most accurate descriptor of the ability to deform under stress.