There isn't a single, universally accepted term for a flat part of the Earth. Here are some possibilities depending on the context:

* Plain: A large, flat area of land, typically with few trees.

* Plateau: A large, flat area of land elevated above the surrounding area.

* Steppe: A large, flat, semi-arid grassland.

* Prairie: A large, flat, grassy area, often in North America.

* Savannah: A large, flat, grassy area, often in Africa.

* Valley Floor: The flat area at the bottom of a valley.

* Mesa: A flat-topped hill with steep sides, common in the southwestern United States.

* Desert: A large, flat area of land with little or no vegetation.

The best term to use depends on the specific characteristics of the flat area you are describing.