drainage basin

A drainage basin is a region of land where water from rain or snow melt drains downhill into a body of water, such as a river, lake, dam, estuary, wetland, sea or ocean. The drainage basin includes both the streams and rivers that convey the water as well as the land surfaces from which water drains into those channels. The drainage basin acts like a funnel - collecting all the water within the area covered by the basin and channeling it into a waterway. Each drainage basin is separated topographically from adjacent basins by a ridge, hill or mountain, which is known as a water divide or a watershed. Especially in North American usage however, watershed refers to the drainage basin itself. Other terms that can be used to describe the same concept are catchment, catchment area, catchment basin, drainage area, river basin and water basin.
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Noun
1. the geographical area draining into a river or reservoir
(synonym) catchment area, catchment basin, drainage area
(hypernym) geographical area, geographic area, geographical region, geographic region

The land drained by a river system. (DOE5)

The area of land that drains water, sediment, and dissolved materials to a common outlet at some point along a stream channel.

The area from which water flows into a stream. Also called a watershed.