Final answer:
An indicator is a substance that changes color in the presence of an acid or a base. It is commonly used in titration experiments.
Step-by-step explanation:
An indicator is a substance that displays different colors when in the presence of an acid or a base. It is a weak acid that ionizes within a known pH range. The protonated form of the indicator molecule predominates in a low pH solution, while the deprotonated form predominates in a high pH solution. The two forms must have different colors to be useful as an indicator. For example, phenolphthalein is colorless in its protonated form and pink in its deprotonated form. Acid-base indicators are commonly used in titration experiments to determine the equivalence point.