wa ‧ ter ‧ shed /ˈwɔːtəʃed $ ˈwɒːtər-, ˈwɑː-/ BrE AmE noun [singular]
[ Date: 1800-1900 ; Origin: water + shed 'line of high ground' (16-21 centuries) (from Old English scead 'separation' ); suggested by German wasserscheide ]
1 . an event or time when important changes happen in history or in your life SYN turning point
watershed in
The 1932 election represented a watershed in American politics.
watershed decision/case etc
a watershed case on pension rights
2 . the (9 o'clock) watershed British English the time in the evening after which television programmes that are not considered suitable for children may be shown in Britain
3 . technical the high land separating two river systems