https://doi.org/10.1351/goldbook.C01191
A reduction in the rate of certain reactions of a substrate RX in solution [by a path that involves a pre-equilibrium with formation of R+ (or R− ) ions as reaction intermediates] caused by the addition to the reaction mixture of an electrolyte solute containing the 'common ion' X− (or X+). For example, the rate of solvolysis of diphenylmethyl chloride in acetone-water is reduced by the addition of salts of the common ion Cl- which causes a decrease in the quasi-equilibrium concentration of the diphenylmethyl cation in the scheme: 
This phenomenon is a direct consequence of the mass-law effect on ionization equilibria in electrolytic solution. More generally, the common-ion effect is the influence of the 'common ion' on the reactivity due to the shift of the dissociation equilibrium. It may also lead to an enhancement of the rate.