noun The topographical boundary dividing two adjacent catchment basins, such as a ridge or a crest.
noun A region of land within which water flows down into a specified body, such as a river, lake, sea, or ocean; a drainage basin.
noun A critical point marking a change in course or development.
watershed moment — watershed event — watershed ruling
noun The time of day after which material of a more adult nature (e.g., violence, swear words, or nudity) may be broadcast on television or radio, either one permitted by law or one contrived from convention (e.g. when children are not watching).
adjective Serving to mark a significant development, change in direction, etc.