To be completely serious, my opinion is:

A folk song is a traditional song. A traditional song is one that's been passed down the generations (let's say about 3 just to stick a number in there) within a specific community, and is known to most members of that community. Calling something a "folk" song is a relatively recent thing to do. I'd imagine the people within the community where they were sung probably just called them songs.

Can you imagine someone saying "Hi, I've just choreographed a nice folk dance I'd like everyone to see." What folk? Where?

The folk process, however, involves singing anything people like. Songs and tunes that people like enough to pass on survive and may (probably will) be changed. No matter how old a song is now, or how many variants exist, it was new at one point, and once had a known author.