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An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically (i.e. non-permanently) when a force is applied to it.
where λ is the elastic modulus; stress is the force that causes the deformation divided by the area to which the force is applied; and strain is the ratio of the change caused by the stress to the original state of the object.
If one specifies how stress and strain are to be measured, that allows for many types of elastic moduli to be defined.