relative hardness

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.
PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1960