The most usual term for large, flat areas is plains.
plains
maria
steppes
They are call Maria ( Latin for "seas" ).
A plain is basically a large, flat area and flood plains are the large flat areas to either sides of rivers. These areas, when it rains so much that the river bursts its banks, are covered in water: flooded.
Both have large areas that are flat and below sea level.
These dark flat areas are called lunar maria, which are large basaltic plains formed by ancient volcanic activity. They have lower elevation compared to the surrounding highlands and were formed billions of years ago by lava flows filling in large impact basins.
When a large flat land is uplifted, various landforms can be created such as plateaus, mountains, or fault-block mountains depending on the type and degree of geological activity. The process of uplift can result in the formation of elevated areas with flat or gently sloping surfaces.
Flat-ish areas near large-ish bodies of water, with moderate climates.
The midwest, or the Great Plains.
Savannas and plateaus are large flat areas of land in Africa that are characterized by grasslands, scattered trees, and relatively low relief. These landscapes are common in various regions across the continent and support a diverse range of wildlife and vegetation.