A base is a substance that donates a pair of electrons to another molecule or ion. When a base is dissolved in water, it undergoes a chemical reaction with the water molecules to produce hydroxide ions (OH-) and other positively charged ions (cations). The hydroxide ions then react with the hydrogen ions (H+) in the water to form water molecules and the conjugate acid of the base.

For example, when sodium hydroxide (NaOH), a strong base, is dissolved in water, it undergoes the following reaction:

NaOH + H2O → Na+ + OH-

The hydroxide ions produced by this reaction then react with the hydrogen ions in the water to form water molecules and sodium ions (Na+):

OH- + H+ → H2O

Therefore, a base can be defined as a substance that increases the concentration of hydroxide ions in a solution when dissolved in water.