Time to revive this thread. Some of the discussion in the "Bullgine" thread is starting to overlap with this one.

Regardless of how we want to define "folk" music, I think we *might* all be able to agree that

1) much instrumental and vocal music circulates (through writing or through hearing, or both) as one-line melody.

2) much of this music is diatonic, containing no accidentals at all, or none but what would be b-flat if the music were transposed to a staff with a key signature showing the key of C. (That doesn't mean the music itself is in the key of C, just that the staff has no sharps or flats in its key signature.)

3) much of this music starts out in a certain key or mode and stays there throughout.

4) much of the music described in (1) (2) and (3) is used for dancing.

5) much of the dance music referred to in (4) is in identifiable genres such as waltz, two-step, reel, jig, hornpipe, strathspey, march.

Much (but by no means all or even most) of the music referred to as "folk" music is has the features enumerated above.