I go by the Steamin' Willie definition.

it works for me.

It winds up a few people who reckon they know a thing or two about folk music.

It is about as good as it gets.

What is it?

I am playing at a folk club tonight. If I were to sing "Tie a yellow Ribbon Round the Old Oak tree" then ergo it is a folk song.

Not as daft as it seems. You see it is a song. Sung in a folk club. Genre is subjective and multi dimensional. (See iTunes for details!)

Got nowt to do with 1954 either.

A song is a song. Common sense and a dictionary would show that to be the case. What word you put front of "song" is where it becomes subjective.   The subject matter of many "folk" songs is the same as many opera, heavy metal, pop, rock, jazz, blues whatever songs.

If I sang a song in Gmaj, in 6/8 time, unaccompanied, about how life is a bit crap, what genre would it be?

The ones I am thinking of include hearing Bruce Springsteen, a bit of Mozart, zillion traditional songs and Blind Lemon Jefferson.

So, when Led Zeppelin sang Gallows Pole, it became a rock song. Live with it. (Don't worry, it's still a folk song because that's the beauty! Rational well adjusted people don't get hung up over it.)