watershed in WordNet English dictionary

1. 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"
   river basin  basin  watershed  drainage basin  catchment area  catchment basin  drainage area 
   
2. a ridge of land that separates two adjacent river systems
   watershed  water parting  divide 
   
3. 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"
   landmark  turning point  watershed