Alec's 1-6 definitions work well. To put it even more simply:

A folk song is a song that comes out of a folk community.

Today the term has become meaningless. So has the term "folk" in relation to music. It hasn't in the case of "folk art". "Folk dance" still has the old meaning. "Folk culture" still works pretty well. "Folk community" -- that still works except in the case of music.

You only need to take a look at a program from a "Folk Festival" here in Australia to see just how meaningless the term "Folk Music" has become.
Cheers from Joy -- a singer-of-old-very-old-and-extremely-old songs.