An idiom is a figure of speech that is a word, group of words or phrase that has a figurative meaning that is not easily deduced from its literal definition.
The word watershed is used literally to mean an area of land that divides the flow of separate rivers, or the high ridge from which water flows into a body of water. It is a division between land and water.
A watershed moment is a turning point, the exact moment that changes the direction of an activity or situation. A watershed moment is a dividing point, from which things will never be the same. It is considered momentous, though a watershed moment is often recognized in hindsight.
Mr Sanchez acknowledged the political influence of an unprecedented strike on International Women’s Day on March 8, when millions of Spanish women abandoned their workplaces and homes and took to the streets, describing it as a watershed moment for social progress. (The Regina Leader-Post)
Aberdeenshire East SNP MSP Gillian Martin said: “This is a watershed moment in the fight against period poverty. (The Press and Journal)
A controversy around wrongly interpreted cervical smear tests marks a watershed moment for the management of Ireland’s health service, a government minister has said. (The Belfast Telegraph)
Weinstein’s victims, his former industry contemporaries, and media figures flooded social media to remark on the watershed moment for the Me Too movement. (Variety)
Start using idioms like this and you will become more fluent!