"watershed" in the noun sense
1. watershed, water parting, divide
a ridge of land that separates two adjacent river systems
2. river basin, basin, watershed, drainage basin, catchment area, catchment basin, drainage area
the entire geographical area drained by a river and its tributaries an area characterized by all runoff being conveyed to the same outlet
"flood control in the Missouri basin"
3. landmark, turning point, watershed
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"
Source: WordNet® (An amazing lexical database of English)
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