Watershed is a term used in the United Kingdom (as well as Canada) to describe a time in television schedules beyond which it is permissible to show a television programmes which have 'adult content', i.e. nudity, explicit sexual intercourse, violence, strong language, etc. In the United Kingdom and the Republic of Ireland, as well as Canada, the specified Watershed time is 9pm.
It is known in the US as "Safe Harbor", and begins at 10pm, though some content (explicit sexual intercourse, for example) is never allowed except on certain restricted-access networks.