My take on the "definitive" aspect would be a song like House of the Rising Sun. This is a song that has its origins somewhere in the murky past, some have suggested as far back as 1600's, and has been brought along the generations in one form or another into today's folk, blues and rock music. I'm sure there are other, undoubtedly better examples; this just seems to be the very essence of folk music - traditional music brought along from generation to generation, morphing as it goes, influencing singers along the way, ending up with an eclectic mix of traditional and modern renderings.