Terrain is "the physical features of a tract of land." In Civilization IV, each tile has one base terrain type, may or may not have hills, and may also have one or more terrain features. Together, these are the tile's terrain, and they are assumed to cover it uniformly. Terrain has a strong effect on the movement and combat of land units. Features of water are also called terrain for our purposes, even though the English parallel to "terrain" would be "marine." Terrain also affects naval combat, although it is far less complex there.