There are mountains and molehills, and we can pretty much tell the difference. But what's the difference between a hill and a mountain? And what's the difference between a mountain and a mountain range? If you recall the 1995 movie The Englishman Who Went up a Hill but Came down a Mountain, the difference was whether the hill was 1,000 feet high or not. But is that from sea level or the surrounding plain? And what if there is no surrounding plain? Tom Scott has a more precise look at the question.
You have to draw the line somewhere, but there's the question of who gets to draw that line. And then there are the various cultures around the world that have designated their hills and mountains before any such regulation in the terms was proposed. Scott has information about the question, but whatever answer you come up with will have exceptions. It depends, in other words.