I always defined Folk as music that evolved, because it was good enough for the next generation to sing it.
People changed the lyrics to fit themselves and the times, because the tune was so at the pulse of the people.
The only other definition, if it sounds like Folk, it is.
I just love the irorny. A community devoted to a style of music that no two members can actually agree on what is. The beauty that is folk. Besides, it makes for great arguements. =)