This is the usual definition but the second part of the definition leads it to be used in many ways.

In the UK in the media world the "watershed" is used to mean - 9pm. I imagine other countries have a similar set of rules but I'm not sure if the term "watershed" is used.

All television programmes before 9pm have to be acceptable for a family audience, after 9pm (increasingly since Channel 5 came along) almost anything goes.

Which leads me to an aside (it is Miscellany after all)- I was wondering at what time "Friends" is shown in the USA? Here it is at 9pm but it is sometimes repeated at 6pm.