Definition

Hallucination

A hallucination is a confident, fluent statement that is false: invented citations, made-up statistics, misattributed quotes. It happens because the model's objective is to predict the most plausible next token, not to be truthful, and benchmarks reward answering over admitting ignorance. The book treats hallucination as a constraint to manage rather than a bug to fix; RAG, evals, and guardrails exist because of it.