Geographical term for a ridge or line that separates drainage basins; also used in environmental and planning contexts.
This mountain range forms the watershed between the Pacific side and the Sea of Japan side.
turning point; watershed moment; moment of truth
Figurative use for a decisive event or period that marks a major change in a situation, career, or history.
That match became the turning point of his life.
This election is a watershed moment that will determine the country's future.