What's Philip's Essential Encyclopedia? Never mind - here's the International Folk Music Council's [in]famous 1954 definition of folk music:

"Folk music is the product of a musical tradition that has been evolved through the process of oral transmission. The factors that shape the tradition are: (i) continuity which links the present with the past; (ii) variation which springs from the creative impulse of the individual or the group; and (iii) selection by the community, which determines the form or forms in which the music survives ... The term can be applied to music that has been evolved from rudimentary beginnings by a community uninfluenced by popular and art music and it can likewise be applied to music which has originated with an individual composer and has subsequently been absorbed into the unwritten living tradition of a community ... The term does not cover composed popular music that has been taken over ready-made by a community and remains unchanged, for it is the re-fashioning and re-creation of the music by the community that gives it its folk character."

In other words, the IFMC defined folk music without saying anything about what it 'expresses' or assuming that the community it derives from corresponds to any identifiable 'national, ethnic or regional culture'. I think they were right.