Here's why:

* Arrhenius Definition: A base is a substance that produces hydroxide ions (OH-) in solution. Sodium hydroxide readily dissociates in water to form sodium ions (Na+) and hydroxide ions (OH-).

* Brønsted-Lowry Definition: A base is a substance that accepts protons (H+). While not as straightforward, hydroxide ions can accept protons to form water.

Let me know if you have any more questions!