Hallucination
Also called: confabulation, fabrication
When a generative model produces content that is fluent and plausible but factually wrong or entirely invented.
A hallucination is output the model presents with the same confidence as correct output, but which has no basis in its inputs or in fact. In medicine this includes invented citations, fabricated laboratory values in a summary, and confidently stated drug interactions that do not exist.
It is not a bug in a specific implementation. It follows from how these models work: they generate likely text, and a likely-sounding falsehood scores as well as a truth.
Where This Gets Misread
The dangerous hallucinations in clinical text are not the absurd ones. They are the ones consistent with the rest of the note ... a plausible dose, a plausible date, a plausible past medical history item. Reviewers catch the absurd ones reliably and the plausible ones poorly, which inverts the usual relationship between error severity and detectability.