Stiffness is the resistance of an elastic body to deflection by an applied force. The stiffness of a body that deflects a distance δ under an applied force P is:

<math>k=\frac {P} {\delta} <math>

Stiffness is typically measured in newton/metre.

As both the applied force and displacement are vectors (respectively P and δ), in general their relationship is characterised by a stiffness matrix, k where:

P = k δ.

The displacement can, in general, refer to a point distinct from that where the force is applied and a complicated structure will not deflect purely in the same direction as an applied force. The stiffness matrix enables such systems to be characterised in straightforward terms.

The inverse of stiffness is compliance, typically measured in units of metre/newton.