The "hallucination" term generally refers to any made-up facts. Harsh as it may be to put this weight of responsibility on LLMs, users of LLMs generally use them in the expectation that what is says is true, and has been (in some magic hand-wavy way) cross-checked or confirmed as factual. Instead they will print out what is most likely to follow the user's input, based on the training data.
Unfortunately, a vast amount of that vast corpus of training data is social media posts which can't be relied upon to be true. But if it gets repeated a lot then it's treated as true in the sense that "what does 'salary' mean" is generally followed by a billion social media posts saying "it referred to the time that Romans soldiers were paid in salt, because salt was a currency at the time"