Arrhenius theorized that a base is a chemical compound that

Arrhenius theorized that a base is a chemical compound that contains hydroxide ions (OH⁻) and ionizes in water releasing them.

Explanation:

Svante Arrhenius was the first scientist to theorize about acids and bases.

Arrhenius' acid  base theory states that an acid is a compound that contains hydrogen atoms and ionizes in water to produce hydrogen ions (H⁺), and a base is a compound that contains hydroxide ions (OH⁻) that are released in water.

These are some examples of compounds which can be named as acid or base following Arrhenius' theory:

This theory is limited because it does not include compounds that do not contain or ionizes producting H⁺ or OH⁻. E.g. NH₃, which is a base (according to other theory).