Check the dictionary that I quoted earlier in this discussion. That fits for me.

"2. Contemporary music in the style of traditional folk music"

I guess you could argue that Joni Mitchell, Donovan or the String Band qualified as folk on that basis. But where does that leave contemporary music that bases itself on the style of Joni Mitchell, Donovan or the String Band? Do we accept that that can be called 'folk' too? If so, where does it stop - or is the definition going to go on expanding indefinitely?