Indicator
Definition
A dye that changes color at a certain pH: the visual signal telling you a titration has reached its endpoint.
Technical definition
An acid-base indicator is a weak organic acid or base whose protonated and deprotonated forms have different colors, changing over a roughly 2 pH unit transition range. It is chosen so its range brackets the titration's equivalence-point pH.
Example
Phenolphthalein is colorless in acid and turns pink above pH 8.2: ideal for a weak-acid/strong-base titration whose equivalence point sits above pH 7.
Common confusion
Indicators do not change color AT a single pH. They transition across a range, so the endpoint is when the color change is first permanent: an approximation of equivalence.