The difference between the values of the absolute hardnesses (η) of a given molecule and the corresponding reference structure (η a): η r = η- η a. The values of relative hardnesses correlate well with the resonance energies of cyclic compounds and may be used as an index of aromaticity.
Source:
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1960 (https://doi.org/10.1351/pac199971101919)