watershed

1. (hydrology, UK, Ireland, Australia, New Zealand) The topographical boundary dividing two adjacent catchment basins, such as a ridge or a crest.Examples: "After Lesjaverk (23 miles) the eastern end of Lesjaskogsvatn is reached. This lake has the distinction of being a watershed. From the east end flows the Lågen, the great river that runs down Gudbrandsdal to Lake Mjøsa, while from the west end the Rauma River emerges to flow down to the Romsdalfjord."Synonyms: water parting, drainage divide, water divide, divideAustraliaIrelandNew-ZealandUK

2. (hydrology, US, Canada) A region of land within which water flows down into a specified body, such as a river, lake, sea, or ocean; a drainage basin.Synonyms: catchment basin, catchment, catchment area, drainage area, river basin, water basinCanadaUS

3. (figurative) A critical point marking a change in course or development.Examples: "In the minds of many, 1847 was a watershed year during which Ceres, Pallas, Juno, Vesta, and Astraea were reclassified as asteroids."; "Coronavirus has been a watershed for the railways. It has accelerated the decline of season tickets and reduced business travel after years of steadily rising passenger numbers."; "In a 2,000-word blog post published ahead of the press event, Brad Smith, Microsoft’s president, called this a “watershed year” and acknowledged the potential downsides, calling for “wide-ranging and deep conversations” on the issues."Synonyms: turning point, crossroadsfiguratively

4. (Canada, British, television, radio) The time during the day which material of a more adult nature (e.g., violence, swear words, or sex) may be broadcast on television or radio, either one permitted by law or one contrived from convention (e.g. when children are not watching).Synonyms: safe harborBritishCanada