Watershed

water + shed, a calque of German Wasserscheide, a compound of Wasser ("water") + Scheide ("divide").

(plural watersheds)

(hydrology) The topographical boundary dividing two adjacent catchment basins, such as a ridge or a crest.
(US) A region of land within which water flows down into a specified body, such as a river, lake, sea, or ocean; a drainage basin.
A critical point marking a change in course or development.
(Canada, British) The time after which material of more adult nature (violence, swear words, sex) may be broadcast on television or radio, either one laid down or one contrived (e.g. when children are not watching...)

(boundary between two adjacent catchment basins) water parting, drainage divide, water divide, divide
(drainage basin) catchment basin, catchment, catchment area, drainage area, river basin, water basin
(time after which adult material may be broadcast) safe harbor (US)
(critical point marking a change in course or development) turning point