An acid-base indicator is a substance that changes color in response to changes in pH, helping to determine whether a solution is acidic or basic. These indicators are often weak acids or bases themselves, and their color change occurs at specific pH levels. Common examples include litmus, phenolphthalein, and bromothymol blue. Indicators are widely used in laboratories and educational settings to test the acidity or alkalinity of various solutions. They can be found in pH test strips or as liquid solutions, providing a simple and effective way to visualize chemical properties without complex equipment.