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 , i.e. ), where is the ionization constant of a m- or p-substituted benzoic acid and 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, , and are typical; also constants which represent dissected electronic effects such as and . For this purpose it might be better always to spell out the term in full, i.e. as 'Hammett sigma constant', and restrict -constants to the scale of substituent constants which is based on benzoic acid. A large positive -value implies high electron-withdrawing power by inductive and/or resonance effect, relative to H; a large negative -value implies high electron-releasing power relative to H.