During an acid-base titration, what observation marks the endpoint?

Core Concept

The endpoint is defined operationally as the instant the indicator shifts color, telling the analyst to stop adding titrant. A well-chosen indicator changes color almost exactly where stoichiometrically equivalent amounts have reacted, which is why endpoint and equivalence point nearly coincide in practice. Separating the observable signal from the underlying stoichiometric condition is a classic exam distinction.