Traditional music is an umbrella term for all folk music that has made it down the generations largely thanks to oral tradition. The obscure and random inception of such songs means that the original author(s) of the song is rarely documented. Rather, the song by time becomes part of a country's or ethnic group's canon of folk songs, usually sung by groups of people performing a custom particular to their culture (ex. pub songs, sea shanties, football anthems, lullabies, children's songs, working songs, dancing tunes etc..).

Countless musicians have performed and re-injected life into traditional songs with examples ranging from Thin Lizzy's (and later Metallica's) Whiskey in a Jar (traditional Irish song) to Bonnie 'Prince' Billy's latest collaboration Among The Gold.

"Traditional" should not be used for the artist name, if the real name of the performing artist or group is known. Rather it should be used for the writer/composer, if they are unknown. The MusicBrainz style guidelines can give you an orientation for better tagging: https://musicbrainz.org/doc/Style/Unknown_and_untitled/Special_purpose_artist