I agree with Frank, although I'm not sure he gave a "definition." "Folk music" seems often to get defined as stuff composed by "folk singers." I don't buy that. We all know songs by Woody Guthrie, and at least one of them, "This Land is My Land," is an outstanding song that will be around a long, long time. But I can't think of even a single reason to say it is any more a "folk song" than (say) "Take Me Out to the Ball Game" or "Sidewalks of New York." Both songs were written to make money. So?

When I was in college, we all sang "Kisses Sweeter than Wine," which I think was written by Pete Seeger, so that everybody knew was a "folk song." Does anybody sing that any more? I haven't heard it in a long, time. It wasn't good enough to last, and it wasn't a folk song by any reasonable definition.

I also suggest that simplicity is an essential trait in a folk song.