The definition is in the process of change. There are traditional folk songs and folk-like
songs in the style of traditional folk songs. Both are now included in the definition.

Usually, the communal theory applies. If people change it through the ages it becomes
like water flowing over a stone. It has a smoothness and beauty of its own.

It's generally in a simple straightforward speech style.

It's generally musically simple without complex harmonies or melodic turns that are
inaccessible to most people. (This would include vocal ornamentation).

I associate it with a history of a particular place or sub-culture.

It is also a photograph of a bird in flight when it is reproduced in a book or in musical notation.

The question becomes when it is a folk-like song, composed by a single author whether this qualifies as the real deal. I really think that it needs to be changed by others along its life span.

It is durable. It lasts because it was important enough for more than one person to keep it alive. It probably may have been changed in this process.

There are some beautiful folk songs that are not known by the general public, these days.
There is an irony here. Music of the "people" not known by the "people" (a large collection of them).

Whether we can correctly define a folk song or not, they will last and be durable because they have some quality that keeps them alive.

Another ID would be that the folk song has many "variants" (variations in tunes and texts found across the nation or world.)

The American cowboy songs "Streets of Laredo" emanating from Ireland as "The Unfortunate Rake" or "Whoopie Ti Yi Yo" from the Irish "Rockin' the Cradle" would be examples of the "variant" theory. "Sam Hall" and "Robert Kidd" turn up in the shape-note hymn "What Wondrous Love Is This?"

The question here is did someone deliberately write a song based on another in a conscious way or was there a process here that defies a planned composition?

We'll be arguing about this for years.

Keeping these songs alive is about singing them, enjoying them and teaching them to
others. The definitions will take care of themselves.

One thing, they are not fads based on contemporary popular music which is ephemeral.
However, there are some fine songwriters who successfully write in a folk-style. The definition might expand to include them. I think we all know who they are. If not,
I would be happy to mention them, some of whom are alive today.

Frank Hamilton