Some feel that reaching for a dictionary makes for dull discussion, but I think usage of terms is the cause of some disagreement here,

'Traditional', in the sense of handed down seems the best term for what people here are mainly interested in. It doesn't exclude songs having started transmission in written form.

It wouldn't do for Roud's book because that includes a lot about the sources from which songs may have been inserted into he handing-on process and mentions people singing from those sources.

'Folk' isn't much use without an adjective. 'Common folk' (as opposed, maybe, to 'rich folk or 'posh folk') seems to be what people mean. Setting aside the need for another adjective I think 'Folk Song' does work for Roud's book in the sense that it is a social history of singing in communities.

However, 'Folk Song' doesn't work for Jim as a title. I think because he wants to restrict the term to music created by the 'common folk'

For me the problem with folk is not 'what is folk?' but 'who are/were folk?' So far as creative origins is concerned one aspect of that is that having a good voice or an ability with words may have allowed someone who prospects were to be an agricultural labourer or factory worker to be a paid performer or broadside seller (or writer). Doing it for money.

(by the way I agree with almost everything Pseudonymous is posting)