While hydroxide is a common component of many bases, particularly those dissolved in water (called Arrhenius bases), there are other types of bases:

* Brønsted-Lowry bases: These bases accept protons (H+). They don't necessarily have to contain hydroxide. Examples include ammonia (NH3) and carbonate ions (CO32-).

* Lewis bases: These bases donate electron pairs. Many Lewis bases don't contain hydroxide. Examples include amines (like ammonia) and ethers.

To summarize:

* Arrhenius bases: Always contain hydroxide (OH-)

* Brønsted-Lowry bases: Can contain hydroxide, but also other groups that can accept protons.

* Lewis bases: A wide variety of compounds, many without hydroxide.

So, while hydroxide is a common characteristic of bases, it's not a requirement for all of them.