Ionic strength is a characteristic of an electrolyte solution (a liquid with positive and negatively charged ions dissolved in it). It is typically expressed as the average electrostatic interactions among an electrolyte's ions. An electrolyte's ionic strength is half of the total obtained by multiplying the molality (the amount of substance per unit mass of solvent) of each ion by its valence squared.

Ionic strength is closely related to the concentration of electrolytes and indicates how effectively the charge on a particular ion is shielded or stabilized by other ions (the so-called ionic atmosphere) in an electrolyte. The main difference between ionic strength and electrolyte concentration is that the former is higher if some of the ions are more highly charged. For instance, a solution of fully dissociated (broken down) magnesium sulfate (Mg⁺² SO₄^-2) has 4 times higher ionic strength than a solution of sodium chloride...