Once upon a time a dictator asked a question that may sound sophisticated to a modern ear, but which is really ridiculous.
Pilate therefore said to him: Are you a king then? Jesus answered: You say that I am a king. For this was I born, and for this came I into the world; that I should give testimony to the truth. Every one that is of the truth hears my voice. Pilate said to him: What is truth? [John 18:37–38; emphasis added]
Why is Pilate’s question ridiculous (setting aside the fact that he only asked it so as to escape the force of what Jesus was saying)? Because if anyone stops for even a second to think about it, he knows exactly what truth is. Truth is correspondence with reality.
The truth of knowledge consists in the conformity of the mind with the thing. [Maritain, An Introduction to Philosophy, 129]
the true is in the intellect in so far as it is conformed to the object understood. [Summa Theologiae I, q.16, a.1]
When we err, we think that something is true when it really isn’t: we falsely suppose that the world is like X, but it’s actually Y. When we lie, we say something that does not correspond to how things actually are despite knowing better. In effect, a lie is a break with reality. Even worse, it amounts to an attempt to induce others to have a false idea of reality.