Parameters of the Kamlet–Taft solvatochromic relationship which measure separately the hydrogen bond donor (α), hydrogen bond acceptor (β), and dipolarity/polarizability (π*) properties of solvents as contributing to overall solvent polarity.
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1132 (https://doi.org/10.1351/pac199466051077)