Specifically the substituent constant for meta- and for para-substituents in benzene derivatives as defined by Hammett on the basis of the ionisation constant of a substituted benzoic acid in water at 25 ^oC, i.e. lg(K_a/K_a^o), where K_a is the ionisation constant of a m- or p-substituted benzoic acid and K_a^o 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, σ⁺, σ^- and σ^o are typical; also constants which represent dissected electronic effects such as σ_I and σ_R. For this purpose it might be better always to spell out the term in full, i.e. as "Hammett sigma constant", and restrict σ-constants to the scale of substituent constants which is based on benzoic acid.

A large positive σ-value implies high electron-withdrawing power by inductive and/or resonance effect, relative to H; a large negative σ-value implies high electron-releasing power relative to H.