What do we call the relatively flat land that extends outward on both sides of a stream or river?

The term for the relatively flat land that extends outward on both sides of a stream or river is "floodplain." Floodplains are areas that are adjacent to rivers and streams, typically characterized by their flat terrain, which allows them to be inundated during periods of high water flow or flooding. These regions play a critical role in the ecosystem by providing habitat for diverse species, allowing the natural deposition of sediments during floods, and acting as natural water filters.

Differentiating this from the other options helps highlight why "floodplain" is the correct term. Wetlands are distinct ecosystems that may be found within floodplains or adjacent to them but are not specifically defined by their proximity to rivers alone. Alluvial plains refer to broader, flat areas created by sediment deposits from rivers but do not specifically denote the area adjacent to a river. Riverbanks are the sides of a river, which can be steep and do not encompass the flat areas that define a floodplain. Understanding these distinctions reinforces the importance of floodplains in river ecosystems and their role in managing floodwaters.