Perhaps "defines" in this context should be looked at less in a musical sense and more in a "culturally iconic" sense? I" suspect it's inevitable that it will be geography/culture-specific, i.e. the same name won't work in the US as works in the UK or Ireland. For that matter the same name probably won't work in England as in Scotland or Wales or Ulster. In this "iconic" sense (and for the US contingent) I'd have to nominate Pete Seeger, less because his music is so definitive, but because for so many people he himself is definitive of the "spirit" of what so many of us think of as "folk" - tradition-based, mostly acoustic stuff not sung by a 'orse.