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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.

Nucleophilicity describes how readily a species forms a bond by donating an electron pair to a specified electrophile. It is a kinetic property, evaluated by relative reaction rates under stated conditions.

It differs from basicity, which concerns the thermodynamic tendency to accept a proton. A bulky base can be effective at removing a proton while reacting slowly at a crowded carbon center. Solvent, charge, polarizability, and steric effects all influence nucleophilicity, so there is no universal ranking valid for every electrophile and medium.