When an AI confidently says something false or made up as if it were true.
A hallucination is when an AI states false facts, wrong numbers, or books and papers that don't exist, as if they were real. It isn't lying on purpose. It picks likely-sounding next tokens instead of looking facts up, so it can string together something smooth but wrong.
It happens most with rare topics, recent news, exact numbers and sources. A confident tone is not evidence.
RAG (giving the AI real documents to read first) and tool use (letting it search or calculate) help, but nothing removes it completely. For anything important, check another source yourself.
Think of it like this
Picture a student who didn't study but won't leave an essay question blank. They fill the space with confident, nice-sounding sentences, and some of it is simply wrong. Neat handwriting doesn't make an answer correct.
For example
Ask for "a study that proves this" and an AI may invent a realistic title and author names.
Common mix-ups
Mix-upWhen AI gets something wrong, it's lying on purpose.
ActuallyLying means knowing the truth and hiding it, but an AI doesn't come with a built-in check for "do I actually know this?". Adding "if you're not sure, say you don't know" to your question can reduce made-up answers, but won't stop them.
Mix-upIf it sounds confident and even gives a source, you can trust it.
ActuallyWhen you're given a source, check three things: search the exact title to see whether it exists, see whether the link opens, and check that the text really says that. Sometimes it names a real book but attaches something the book never says.
Try it in class
For teachers at the start of a lesson — or for students working on their own.
- Question
- How can you pick out the parts of an AI answer that might be hallucinated?
- Activity
- Ask an AI something it probably saw little of in training — say, the history or recent news of your own town. Highlight every number, date, name and source in the answer. Check each one against another source (a textbook, an official website, or a record on this site that links its source) and sort them into right, wrong and couldn't find. You can also run years and dates through "Is it true?".