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The most usual term for large, flat areas is plains.
plains
savannahs
maria
Abyssal Plain. :)
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.
Flat-ish areas near large-ish bodies of water, with moderate climates.
The midwest, or the Great Plains.
Back when they were named that, we didn't know that.