Parameters to measure separately the ability of a solvent to enter into non-specific solvent–solute interactions (permittivity ɛ and refractive index η D) and specific solvent–solute interaction (solvent basicity or nucleophilicity B and solvent acidity or electrophilicity E) as contributing to overall solvent polarity.
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1134 (https://doi.org/10.1351/pac199466051077)