Folk" has a widely acknowledged core of specific titles like "John Henry," "The Seeds of Love," "Lord Randall," and "The Irish Washerwoman," to name a very few.

"Folkness" is a mostly (but not entirely) subjective quality shared by such pieces.

How one defines "folk" in general depends on how much kinship is thought to exist between such examples and other titles under consideration. And outside academia (and sometimes within it) the criteria of "kinship" are wildly subjective.

"Folk" was once a useful label for distinguishing "anonymous music and song of the people" from formal, attributed compositions that were performed according to the published sheet music.

For various reasons that basic distinction has been lost.

"Folk" is still a useful term, but without specific context it covers multitudes and can be downright misleading.

So caveat emptor.