A lie is a false statement made intentionally. (Merriam-Webster: “Lie, verb: to make an untrue statement with intent to deceive.”) In some cases, it’s safe to say Trump is intentionally trying to deceive. In other cases, it’s far less clear that he’s being wrong intentionally — because, with Donald Trump, you regularly can’t rule out the possibility that he is confused or ignorant.
If we journalists are going to present ourselves as arbiters of truth, we have to stick to what we know is true. And that means not calling something a lie when we don’t have a reasonable certainty that Trump’s intention is deception.