Terrain is the natural features of a piece of land, such as whether it is flat, hilly, rocky, or swampy. It affects how easy it is to travel, build, or live there.
From French *terrain* 'ground, land', from Latin *terra* 'earth, land'. It originally simply meant 'dry land' as opposed to the sea.
Terrain is like a land personality: some land is friendly and flat, some is wild and jagged. The same root *terra* hides in 'territory', 'terrace', and even 'Mediterranean'—the sea 'in the middle of the lands'.
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