The bending stiffnessEI of a beam (or a plate) relates the applied bending moment to the resulting deflection of the beam. It is the product of the elastic modulusE of the beam material and the area moment of inertia I of the beam cross-section. According to elementary beam theory, the relationship between the applied bending moment M and the resulting curvature κ of the beam is
where w is the deflection of the beam and x the spatial coordinate. In the literature sometimes the above definition is given with a minus sign depending on convention.