In general it is called a "plain" (not to be confused with mathematical plane). If it is an area raised above its surroundings it is a plateau. More types of plains in the link below.
a broad area of flat or gently rolling land is called a plain.
plateu
plateu
A large piece of flat land is called a plain. A desert is a dry, barren area with little vegetation. A mountain is a raised landform with a peak or summit.
A large flat area of land usually without many trees is called a plain or a prairie. Plains have grasslands and few trees, while prairies are large expanses of flat grassland.
butte, mesa or plateau
it is called a plateau.
The large, flat center of Mexico is called a plateau. This area is also called the land of many climates because of the constantly changing weather pattern in the area.
A large piece of flat land is called a plain. A mountain desert refers to a desert area with significant mountainous terrain.
Plateau
A large, flat area of land is called a plain.
A Plateau