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.

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.

A watershed is an area of high ground which divides two or more river systems, so that all streams on one side flow into one river and those on the other side flow into a different river.