Stiffness's the resistance of an elastic body to deformation by an applied force. It's an extensive material property. Definition The stiffness, k, of a body's a measure of the resistance offered by an elastic body to deformation (bending, stretching or compression). » k=frac where » A's the cross-sectional area, E's the (tensile) elastic modulus (or Young's modulus), » L's the length of the element. For the special case of unconstrained uniaxial tension or compression, Young's modulus can be… (More on Stiffness)