A mountain is a large and often steep landmass that's much higher than the surrounding area. It's not to be confused with a molehill.
The word mountain has been around since the 1200s. A mountain is much larger than an average hill; it's a mass of land that rises up to 2000 feet or so. Mountain does not always describe a land mass. It can also be used to refer to an undertaking, a pile of something, or even a large dose of trouble. You might have a mountain of mashed potatoes you don't want to eat, or a mountain of homework.