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Stiffness is about how much a material resists deformation under load, especially while remaining in the elastic range where it would return to its original shape if the load is removed. In that elastic region, stress and strain are proportional, and the constant that links them is the Modulus of Elasticity (also called Young's modulus). This value directly measures stiffness: a higher modulus means the material is stiffer and will deform less under the same applied stress. For example, steel has a high modulus and is relatively stiff, while rubber has a low modulus and deforms readily. Other properties describe different behavior: ductility is how much plastic deformation the material can undergo before fracture, yield strength is the stress at which plastic deformation begins, and toughness is the energy the material can absorb before fracture.