Definition of "watershed"

In the minds of many, 1847 was a watershed year during which Ceres, Pallas, Juno, Vesta, and Astraea were reclassified as asteroids.

2009 , David A. Weintraub, Is Pluto a Planet?: A Historical Journey through the Solar System, Princeton, NJ: Princeton University Press, page 115

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.

2021 November 17, Anthony Lambert, “How do we grow the leisure market?”, in RAIL, number 944, page 34

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.

2023 February 7, Cade Metz, Karen Weise, “Microsoft Throws a Coming-Out Party for A.I.”, in The New York Times‎

Green's Dictionary of Slang is a watershed publication in the annals of slang lexicography, being, beyond doubt, the most comprehensive scholarly dictionary of slang ever published.

2018, James Lambert, “Anglo-Indian slang in dictionaries on historical principles”, in World Englishes, volume 37, page 251