a ridge of land that separates two adjacent river systems
"The new highway construction was halted when workers discovered an ancient watershed beneath their equipment."
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"
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"
The topographical boundary dividing two adjacent catchment basins, such as a ridge or a crest.
"The team climbed to the highest point of the hill, which served as the watershed separating the river's drainage from its neighbor's basin."
In plain English: A watershed is an area of land where all rainwater drains into a common river, lake, or ocean.
"The movie marked a watershed moment in his career when he finally won an award for best actor."
Usage: Use the noun form when referring specifically to a mountain ridge that separates river drainage areas. Use the adjective "watershed" only for metaphorical situations describing a pivotal turning point in history or personal life.
Serving to mark a significant development, change in direction, etc.
"The election of 1964 was seen as a watershed moment that shifted American politics toward the right."
In plain English: A watershed is an important turning point that marks when something changes completely for better or worse.
"The watershed moment in the election changed everything for the country."
The word watershed is a direct translation from the German term Wasserscheide. It originally referred to a ridge that separates two river basins before taking on its modern meaning in English.