Second-order Stark effect is analysed in terms of the third-order non-linear susceptibility chi((3))(omega; omega,0,0) within a centrosymmetric one-dimensional three-level model. The Stark effect manifests itself in the electroabsorption spectrum mainly as the first or the second derivative of the unperturbed absorption spectrum that corresponds to either Stark shift or Stark broadening, respectively. Domination of the first or the second derivative in the electroabsorption lineshape depends on the energy gap between the first dipole-allowed state and a nearby excited dipole-forbidden state. The model takes into account inhomogeneous broadening and vibronic sidebands of both excited states.