A substance that can be added to the reaction mixture to indicate the equivalence point of the titration. Colored indicators are often used to measure pH. These indicators are a different color in a basic environment than in an acidic environment. Litmus, for example, is blue in a basic environment and red in an acidic environment.
Indicators are weak acids or weak bases (often derived from plant pigments) that have distinctly different colors in their ionized and non-ionized forms and which are related to the pH of the solution being tested.
For example, the use of a chemical named phenolphthalein as an indicator for a titration in which a strong base is used as the titrant and acid as the substance titrated would give a color change from colorless to pink in the reaction flask near the equivalence point.