Watershed
Etymology
A calque of the German Wasserscheide, "water divide," combining Wasser, "water," with scheiden, "to part, divide." Geographically, a watershed is the ridge of high ground separating rivers that flow into different basins — rain falling on one side runs one way, rain on the other side the opposite way. That image of a line after which everything diverges became the metaphor for a historical turning point.
noun · /WAW-ter-shed/
A turning point that marks an important division between what came before and what follows.
What it names
The word for a decisive dividing line in time — a watershed election, a watershed moment — after which things flow in a clearly different direction.
Example
The strike proved a watershed in labor history; nothing about the industry’s bargaining looked the same afterward.
Related idea
Cusp is the edge of a transition you are poised on, often before it happens; a watershed is the dividing point itself, usually recognized once you can see the different course things took after it.
noun · /WAW-ter-shed/
The area of land whose streams all drain into one river or sea, or the ridge dividing one such basin from the next.
What it names
a geographic drainage system — a catchment area, or the high line of land that parts the waters
Example
Rain falling on this side of the ridge stays within the river’s watershed.
Related idea
The literal dividing line gave us the figurative turning point: a watershed moment parts “before” from “after” as a ridge parts one river’s waters from another’s. In British broadcasting, the “watershed” is the evening hour before which adult content may not be shown.