Drainage basin is a geographic term about rivers. It is also called catchment, catchment area, catchment basin, drainage area, river basin, and water basin. It is an area of land. All water that falls on that land flows into one river. It can flow directly into the river or go through tributaries (smaller rivers that flow into the bigger river) first. A watershed is a line separating two drainage areas.
River basins are an open system with inputs and outputs of water