Heavyset_go wrote a good answer from one point of view, I would like to add another. Namely that "hallucination" the word means quite different things for a hum

Heavyset_go wrote a good answer from one point of view, I would like to add another. Namely that "hallucination" the word means quite different things for a human and an llm. For a human a hallucination roughly refers to a sensory experience where you experience something that isn't there. For an LLM it refers to making something up when answering a question. A closer phenomenon to that would be a false memory, but even that isn't quite the same thing. The thing is that we want LLM's to learn patterns, we want them to generalize from its training data to a certain extent. Hallucinations are basically undesirable instances of this. The user expects to hear about somethign that does exist, but the answer they get is rather about somethign that the LLM has extrapolated might exist.