watershed

noun

1. a ridge of land that separates two adjacent river systems

   noun.location

   Synonyms divide water parting

   Source: WordNet 3.0

2. the entire geographical area drained by a river and its tributaries; an area characterized by all runoff being conveyed to the same outlet

   noun.location

   Synonyms basin catchment area catchment basin drainage area drainage basin river basin

   Source: WordNet 3.0

3. an event marking a unique or important historical change of course or one on which important developments depend

   noun.event

   Examples

   The agreement was a watershed in the history of both nations.

   Synonyms landmark turning point

   Source: WordNet 3.0

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

   noun

   Source: webster1913