Specifically the substituent constant for meta- and for para-substituents in benzene derivatives as defined by Hammett on the basis of the ionization constant of a substituted benzoic acid in water at $25{}^{\text{⚬}}\text{C}$, i.e. ${\log }_{10}(\frac{K_{\text{a}}}{K_{\text{a}}^0})$, where $K_{\text{a}}$ is the ionization constant of a m- or p-substituted benzoic acid and $K_{\text{a}}^0$ that of benzoic acid itself. The term is also used as a collective description for related electronic substituent constants based on other standard reaction series, of which, ${\sigma }^{+}$, ${\sigma }^{-}$ and ${\sigma }^0$ are typical; also constants which represent dissected electronic effects such as ${\sigma }_I$ and ${\sigma }_R$. For this purpose it might be better always to spell out the term in full, i.e. as 'Hammett sigma constant', and restrict $\sigma$-constants to the scale of substituent constants which is based on benzoic acid. A large positive $\sigma$-value implies high electron-withdrawing power by inductive and/or resonance effect, relative to H; a large negative $\sigma$-value implies high electron-releasing power relative to H.