Basicity

Basicity measures a chemical species' tendency to act as a base. For proton transfer, it is the tendency to accept a proton; for a Lewis base, it concerns association with a Lewis acid through an electron pair. A comparison must specify the reaction partner and medium.^[ref]

For a base $B$ in water,The $a_i$ are thermodynamic activities, with the solvent convention absorbed into $K_b$. At the same temperature and standard-state conventions, $K_a(B{H}^{+})K_b(B)=K_w$. Thus a higher conjugate-acid $p{K}_a$ corresponds to stronger basicity in that medium. Basicity is an equilibrium property; a fast nucleophilic substitution need not identify the stronger equilibrium base.