离子强度是溶液中离子浓度的量度，是溶液中所有离子浓度的函数，定义如下：

其中：

离子化合物溶于水中时，会解离成离子。水溶液中电解质的浓度会影响到其他盐类的溶解度。尤其是当易溶的盐类溶于水中时，增强难溶盐类的溶解度。而影响的强弱程度就称为离子强度。

例如，0.050 mol·dm Na₂SO₄（硫酸钠）和0.020 mol·dmNaCl（氯化钠）混合溶液，其离子强度为：

I = 1/2((2 × (+1) × 0.050) + (+1) × 0.020 + (−2) × 0.050 + (−1) × 0.020) = 0.17 mol·dm

The concept of ionic strength was first introduced by Lewis and Randall in 1921 while describing the activity coefficients of strong electrolytes. The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as the dissociation or the solubility of different salts. One of the main characteristics of a solution with dissolved ions is the ionic strength.