Watershed

n. ridge or high area dividing land drained by different rivers or systems; area which drains into a river; point of change, turning point

Watershed originally meant the dividing line between two drainages, shedding water in either direction. It has come to refer to:GeographyA drainage water divide (especially in Britain and other Commonwealth countries), meaning the ridge of land that separates two adjacent drainage basins.In North American usage, a drainage basin or river catchment, meaning the region of land whose water drains into a particular watercourse.The European Watershed is the line which divides the drainage basins of the major rivers of GermanyThe Humber Watershed is a drainage basin of south-central Ontario, Canada.Watersheds of Indiana, consist of six distinct Indiana drainage basins that drain to five major bodies of water.Taunton River Watershed, a drainage basin containing 562 miles of rivers, lakes, ponds, streams, and wetlands in southeastern Massachusetts.You can also see a list of watersheds or drainage basins.
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Noun
1. a ridge of land that separates two adjacent river systems
(synonym) water parting, divide
(hypernym) line
(hyponym) continental divide
2. an event marking a unique or important historical change of course or one on which important developments depend; "the agreement was a watershed in the history of both nations"
(synonym) landmark, turning point
(hypernym) juncture, occasion
(hyponym) Fall of Man

(n.)
The whole region or extent of country which contributes to the supply of a river or lake.
  

(n.)
The line of division between two adjacent rivers or lakes with respect to the flow of water by natural channels into them; the natural boundary of a basin.
  

The land area that drains into a stream; the watershed for a major river may encompass a number of smaller watersheds that ultimately combine at a common point.