The role of water is essential in this definition. Pure solids like NaOH must be dissolved in water to act like a base. There are many different bases, but they have something in common. 1- and a metal. This last idea makes it easier to identify bases.
Bases have a hydroxide ion combined with a metal.
You should see a common characteristic in these bases. They all have a metal element and a hydroxide ion. The reason these substances are bases is that they dissolve in water to release hydroxide ions. The limitation on these compounds is their solubility in water. Typically all metal hydroxides are bases.