等电点是一个分子或者表面不带电荷时的pH值。

被称为两性离子的两性分子同时含有带正电荷和负电荷的官能团。整个分子的总电荷则由其周围环境的pH值决定，根据pH值的不同整个分子可能带正电荷，也可能带负电荷。其原因是因为这样的分子在不同的pH值环境中可能会吸收或者丧失质子（H）。在pH值等于等电点时这样的分子所带的正电荷和负电荷互相抵消，使得整个分子不带电。

表面也会自然地带电荷形成固定层。一般假如决定表面电荷的离子是H/OH的话那幺表面浸入的液体的pH值也会决定表面的总电荷。在这里等电点也是表面总电荷为零时的pH值。

The isoelectric point (pI, pH(I), IEP), is the pH at which a particular molecule carries no net electrical charge. The standard nomenclature to represent the isoelectric point is pH(I), although pI is also commonly seen, and is used in this article for brevity. The net charge on the molecule is affected by pH of its surrounding environment and can become more positively or negatively charged due to the gain or loss, respectively, of protons (H).