Ratio of an applied force or torque to change in position for a mechanical system.

The stiffness of a system with respect to a deformation (e.g., the stiffness of a spring with respect to stretching) is the second derivative of the energy with respect to the corresponding displacement; this measures the curvature of the potential energy surface along a particular direction. Positive stiffness is associated with stability, and a large stiffness can result in a small positional uncertainty in the presence of thermal excitation. Negative stiffnesses correspond to unstable locations on the potential energy surface. Alternative terms for stiffness include force gradient and rigidity.

A quality of paper alluding to its rigidity and resistance to bending and inflexibility.

The ratio of change in load to accompanying change in deformation of a member fabricated of a Hookean material and activated within its elastic limit; the slope of the load-versus-deformation plot beneath the elastic limit.

Inability (as opposed to unwillingness) to flex the joints or stretch the musculature to the degree and in the way required to perform the task at hand (often confused with "Tense" or "Resistant.")

state of limbs which have become stiff due to the cold or due to being in a restricting position for too long. It arises during long winter rides, or on uncomfortable motorcycles. In extreme cases, stretching a stiff leg can result in an unexpected inertia which may cause the rider to fall when he stops! Riders should stretch their limbs frequently, even on the motorways.

A kart frame is designed to allow flexing, a very important property of the frame. The stiffness of the frame is a measure for how easily it can be flexed. Fast classes which race with sticky tires require a stiffer frame than slower classes which utilize harder tires. Stiffness can be changed by the setting of the torsion bars and the thickness of the rear axle.