When our bunch do gigs for pay, we label songs or melodies "traditional" when they are such that no copyright can be claimed on them. Some would have it that they must not have a known author, but if that was so much of the popular Burns material couldn't be called "traditional"--and to me that is patently nonsense. In the truest sense of the word, as has been said already, "traditional" means the passing of the material from generation to generatio; in other words, folk-memory.