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

A large area of flat land that is elevated high above sea level is known as a plateau. Plateaus are characterized by their significant elevation compared to the surrounding areas and their relatively level surfaces. They can be formed through various geological processes, including volcanic activity, erosion, or the uplifting of the Earth's crust.

In contrast, a plain refers to a flat, low-lying area of land that is typically close to sea level and does not have the same elevation associated with plateaus. A range refers to a series of mountains or hills, while a hill is a rounded elevation that is lower than a mountain and does not possess the extensive flatness and elevation characteristic of a plateau. Thus, the definition of a plateau fits perfectly with the description in the question.