A hallucination is when an AI system states something false with full confidence — inventing sources, dates, functions, or facts. It happens because the model generates plausible text, not verified truth.
Hallucinations are the core reliability problem of generative AI. Any workflow that matters needs a verification step — sources, tests, or human review.
A model cites a research paper with a convincing title and author list — but the paper does not exist. The citation sounded right because it fit the pattern.