Merriam Webster -
Main Entry: folk song
Function: noun
Date: 1847
: a traditional or composed song typically characterized by stanzaic form, refrain, and simplicity of melody


Random House:
folk music
–noun 1. music, usually of simple character and anonymous authorship, handed down among the common people by oral tradition.
2. music by known composers that has become part of the folk tradition of a country or region.

(a great case can be made for contemporary folk songs having entered the tradition of a country or region - or a community)