## relative hardness

https://doi.org/10.1351/goldbook.RT07092
The difference between the values of the absolute hardnesses ($$\eta$$) of a given molecule and the corresponding reference structure ($$\eta _{\text{a}}$$): $$\eta _{\text{r}} = \eta - \eta _{\text{a}}$$. 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)