The most usual term for large, flat areas is plains.
plains
Large stretches of very flat land without trees are typically called plains. These areas have low vegetation and are often used for agriculture due to their fertile soil. Examples include the Great Plains in North America and the Eurasian Steppe in Europe and Asia.
maria
Europe is a huge place (a continent) it contains areas with flat planes, others below sea level as well as areas of hills and mountains (the Alps).
Eastern Europe is mainly flat with some hills, although there are a few large mountain chains.
a large area of flat unforested grassland in southeastern Europe or Siberia.
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.
The Przwalskis horses live in europe and asia mostly found in plains or a larg amount of flat land
A vast, flat, treeless Arctic region of Europe, Asia, and North America in which the subsoil is permanently frozen.
Eastern Europe is mainly flat with some hills, although there are a few large mountain chains.