Definition
A hallucination is a confident statement a model makes that is not supported by any source. For brands the common form is not invention from nothing but a plausible wrong detail: an outdated price, a feature that belongs to a competitor, a market you left.
Why it matters
A wrong answer about your brand does more damage than no answer, and it is repeated to every buyer who asks. It is also the most fixable failure, because it usually traces to one stale source.
How to measure it
Keep the raw answer text, not only a score, so wrong claims can be read and traced. Then follow the citation to the source that carries the stale fact and fix it there.