1. Classical MechanicsStiffness is the extent to which an object resists deformation in response to an applied force. The complementary concept is flexibility or pliability: the more flexible an object is, the less stiff it is. == Calculations == The stiffness, k , {\displaystyle k,} of a body is a measure of the resistance offered by an elastic body to deformation. For an elastic body with a single degree of freedom (DOF) (for example, stretching or compression of a rod), the stiffness is defined as k = F δ {\displaystyle k={\frac {F}{\delta }}} where, F {\displaystyle F} is the force on the body δ {\displaystyle \delta } is the displacement produced by the force along the same degree of freedom (for instance, the change in length of a stretched spring) Stiffness... from wikipedia.org