Here's why:

* Arrhenius definition of a base: A base is a substance that produces hydroxide ions (OH-) when dissolved in water.

* Brønsted-Lowry definition of a base: A base is a substance that can accept a proton (H+). Hydroxide ions readily accept protons to form water (H2O), fulfilling this definition.

While other ions can be present in bases, the hydroxide ion is the defining characteristic of a base.