In order to understand what constitutes a lie, we must first understand what truth is. However, this is near impossible because the two are synonymous. When asked what truth is, one could say that it "is not a lie". However, when asked what a lie is, one could also say "something that is not the truth". In order for something to become truth, it needs to be universally socially acceptable.
And yet, even that law isn't failproof. What happened when, in the earliest of days, men came to the conclusion that the world was flat? This was universally accepted as a fact when in reality, it was not. So who can decide what constitutes truth or fiction?
What is a truth, but a lie agreed upon?