A large area of flat land elevated high above sea level is called a

A large area of flat land, that is at a high altitude is called a plateau. An excellent example of a plateau is Tibet.

The answer is - plateau.

The plateaus are relatively flat lands that are found on a higher altitude. The altitude can be anywhere from 600 meters above sea level and up to 4000 + meters above sea level. The reasons for their formation can be different, it can be from collision of tectonic plates and during the mountain formation in certain parts to form a flatter part, erosion, volcanic activity etc. Occasionally people confuse valleys with lower plateaus, especially in countries that are mostly on higher altitude because they seem like lowlands compared to the surrounding area. The best known plateau in the world is probably the Tibetan Plateau which is a consequence of the collision of the Eurasian and Indian continental plates, but there's also some relatively unknown plateau like Vitachevo that is a consequence of a volcano eruption.