Resonance energy of a resonance stabilized species is the difference between its energy and the energy of its lowest-energy resonance form.
eg:
The hypothetical, resonance-stabilized molecule X has three resonance forms (X1, X2, and X3). The graph below shows the relative energies of X (hybrid) and its resonance forms.
The difference in energy between X and X3 is the resonance energy of X.
