https://doi.org/10.1351/goldbook.K03367
Parameters of the Kamlet–Taft solvatochromic relationship which measure separately the hydrogen bond donor ($\ce{\upalpha\!}$), hydrogen bond acceptor ($\ce{\upbeta\!}$), and dipolarity/polarizability ($\ce{\uppi\!^{\ast}}$) properties of solvents as contributing to overall solvent polarity.