Stiffness is the ratio of change of force (or torque) to the corresponding change in translation (or rotational) deflection of an elastic element. Stiffness is often defined with respect to static or dynamic loading conditions.
When a force is applied to a structure, there is a displacement in the direction of the force; stiffness is the ratio of the force divided by the displacement. High stiffness means that a large force produces a small displacement. When discussing the stiffness of a material, the concept is the same, except that stress substitutes for force, and strain substitutes for displacement.
|1 lbf/in||=||17.5 Nm-1||1 lbf/µin||=||175 Nµm-1|