A hallucination is when AI says something untrue with a straight face. Because a large language model works by predicting fluent text, not by looking facts up, it can produce a clean, confident answer that is simply wrong: a made-up quote, a fake statistic, a source that does not exist.
The fix is a habit, not a setting: treat AI as a fast drafter, not an oracle. Check anything factual, and switch on web search for current facts so Claude can cite real sources.