Definition
A hallucination is false or unsupported content generated without an adequate basis in the model’s input or available evidence. It can take the form of an invented fact, citation, event, API, or relationship between real facts.
The term describes a problem in the generated output. It should not become a label for every incorrect result produced by an AI application.
Simple example
A support assistant receives a policy that allows returns within 14 days. It answers that customers have 30 days and invents an exception for damaged items. The 30-day claim contradicts the supplied policy, while the exception has no support in the evidence available for the answer. Both are hallucinations under this source-grounded definition.
Now consider a different case. The supplied policy itself says 30 days, but that document is obsolete. If the assistant reports the 30-day rule accurately, the answer is wrong for the customer, but the immediate failure is stale source data rather than unsupported generation.
Why it matters
Calling every bad answer a hallucination sends the investigation in the wrong direction. An unsupported claim may call for better grounding or an explicit option to say that the available evidence is insufficient. Stale data needs to be corrected at the source. Missing evidence may require investigating source coverage or retrieval, while failed calculations and tool calls require separate investigation.
The distinction also affects measurement. A single correctness score cannot show whether the model invented a claim, repeated bad evidence, or received the right information and used it incorrectly.
One important nuance
“Hallucination” does not have one universally applied technical definition. Some teams include any false model output. Others reserve the term for claims that lack support in the model’s input or designated evidence. Define the term in an evaluation plan before counting hallucinations.
Faithfulness checks whether generated claims are supported by supplied evidence. Factual consistency can be assessed against source material or independently established facts, depending on the evaluation. Agreement with a supplied reference does not establish that the reference reflects the real world.