Real Dictionary

watershed
If something such as an event is a watershed in the history or development of something, it is very important because it represents the beginning of a new stage in it.
countable noun: The watershed is a time before which television broadcasters have agreed not to show programmes unsuitable for children, for example programmes that contain scenes of sex or violence.

watershed in American English
a ridge or stretch of high land dividing the areas drained by different rivers or river systems
adjective: constituting a watershed
noun: the ridge or crest line dividing two drainage areas; water parting; divide

watershed in British English
a watershed
noun: the dividing line between two adjacent river systems, such as a ridge