Or let's try it this way: "folk" is what happens when the record industry isn't looking.

Moses Asch – Folkways Records – Woody Guthrie.

Folk is a genre-category-type-style-aisle-shelf-breed-[X]... but a 6th century German-Irish rowing chant isn't goombay, junkanoo or calypso and none of any of it is popular in British folk clubs.

Except for not knowing what it is; what critical atribute one wants to measure and how to measure same... we calculate it's revival comes every thirty (30) years. Cue Tom Lehrer's New Math.