Lies

What I would like to know is what defines a lie? Isn't truth relative? Certainly, there are outright lies, "Oh, I mailed that check yesterday", "We'll be in touch with you regarding the job offer", etc. But what about things that are written in history books that are later proven to be false, whether mistold by the author, or the government, or whomever, or whether they were not truly "lies", but rather, "fables" passed on to teach a lesson rather than to reveal the truth in the story?