1. Classical MechanicsStiffness or rigidity is the extent to which an object resists deformation in response to an applied force. The inverse of stiffness is flexibility or pliability: the more flexible an object is, the less stiff it is. Other terms with related meanings are compliance and elasticity. == Calculations == The stiffness, 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=Fδ where, F is the force on the body δ is the displacement produced by the force along the same degree of freedom (for instance, the change in length of a stretched spring) Stiffness is usually defined under quasi-static conditions, but sometimes under dynamic loading. In the International System of Units, stiffness is typically measured in newtons per meter (N/m). In Imperial units, stiffness is typically measured in pounds (lbs) per inch. Generally speaking... from wikipedia.org