BehaviorsEstablished
Hallucination
noun
When an AI makes something up or gets a fact wrong and presents it as if it were true.
What does hallucination mean?
You ask for a source, and the chatbot gives you a title, an author and a year. It all looks right. Then you search for it and the book doesn't exist.
That's a hallucination: false or fabricated information presented as fact. It might be a fake citation, an invented quote, a wrong date or a feature that a piece of software never had. What makes hallucinations especially risky is that they can sound just as convincing as correct answers.
How people use it
It cited a court case with a real-sounding name and docket number. The case doesn't exist. Classic hallucination.
Example written for this dictionary.
The word is often used loosely for almost any factual error an AI makes. More narrowly, it usually refers to information that the model generates without a reliable basis, especially when details are invented rather than simply mistaken.
The boundary isn't always clear. Even so, "check for hallucinations" has become a common instruction when people use AI for research or other fact-heavy work.
Why it matters
A false answer can be easy to believe when it is written clearly and confidently. That makes verification an important part of using AI for anything where accuracy matters.
The word itself is also debated. Calling the behavior a "hallucination" can make it sound as though the model is having an experience similar to a human one, which is not what the technical term is meant to imply.
Where it came from
Researchers were using "hallucination" for machine-learning outputs before chatbots became widely popular, but the term became much more familiar after the rise of ChatGPT and other generative AI systems.
Cambridge Dictionary chose "hallucinate" as its 2023 Word of the Year and added an AI-related meaning to the entry. Some critics dislike the term because it can suggest that a model has a mind that perceives things. They prefer alternatives such as fabrication or confabulation.