This is going to look like a complete reversal of my earlier stance.
First of all there is nothing wrong with the Sao Paulo definition of 1954. It is very precise and clear and whether you agree with it or not, at the time it was a widely accepted definition by anyone who was taking the music seriously. It largely followed the descriptions Sharp had made in 'Some Conclusions' but making the boundaries clearer.

The wider uses of 'folk song' are of course, like other genres not definable, but they can easily be described. I see nothing wrong in using qualifiers like 'traditional', 'contemporary'. These are words that have been used on the folk scene throughout the second revival in the British Isles, and are easily understood by the vast majority of us.

Jim objects to recent pop songs being sung in folk clubs. I think I would too if I went to a folk event and found that the majority of what was sung was pop songs. I can think of one well-known professional who regularly throws in 2 or 3 pop songs from his youth into his act. I put this down to him being true to his own influences.
It has never put me off. He is very entertaining and we all know and can join in with the choruses.

As for electric folk, folk rock, or any similar genres, these have brought many more young people into the music than Fred or Harry or Walter or the Copper Family ever did.