Here are some definitions that people have suggested (bet you didn't know that was what you were doing).

A "folk song" is...

- any song sung in a folk club
- any song sung by a folk singer & accepted by a folk audience
- any song widely sung in folk clubs & taken up on the folk scene
- any traditional song, plus new songs in traditional idioms
- any traditional song but no newly-composed songs, unless they are taken up by ordinary people in the way that traditional songs were

I think an awful lot of confusion - and heat - has been generated by people applying the word 'folk' to the performer and starting from there. If somebody thinks of himself as a folk singer - & gets bookings from other people who also think he's a folk singer - it's understandable that that person would be a bit narked to be told that little or none of what he's singing is actually 'folk'.

If you get away from the idea that the adjective 'folk' applies to the singer - and apply it to individual songs - then the problem goes away. So Martin Carthy's a singer, Ewan MacColl was a singer, Seth Lakeman's a singer. Carthy sings mostly folk songs; MacColl sang folk songs and his own songs, most of which were in a folk idiom; Lakeman sings some folk songs and a lot of his own, some of them in a folk idiom.

This is assuming that there's something about the song itself which makes it 'folk' - as distinct from, say, 'rugby club songs' or 'Boy Scout songs', which are whatever songs are actually being sung by rugby players or Boy Scouts. Admittedly, there are 'folk club songs', or standards as they'd be called in the jazz world: songs you'll never hear on the radio but can be sure of hearing in any folk club if you wait long enough - "No Man's Land", "Beeswing", "Sally free and easy", "Farewell to the gold"...

As for what it is that makes a folk song a folk song, in one word: origin. Not the ultimate origin but the last stop, as it were, before it enters the repertoire of professionals and hobbyists. I'd suggest that folk songs are songs that have been collected from people who didn't know where they came from; songs that people sing for fun, round the fire or while they're working*. Which means that folk song exists mainly in the past tense - singing songs in that way isn't something people do much any more. A new song would become a folk song if it got away from its composer and lived on in that way, but it's not likely to happen now.

I think this is what most of the collectors would have understood by 'folk song', however imperfect some of their collecting practice was*. But I know a lot of people really don't like using the word 'folk' as restrictively as this, so I don't suppose there's much chance of going back to it.

*Dave Harker showed that some of what we now consider the traditional repertoire probably wasn't sung like this, but was faked up by collectors or their contributors. (Not that he was the first to go down this road; Robert Chambers cast doubt on several of the big ballads, including Sir Patrick Spens, as early as 1849.) But what Harker didn't show (although you'd never know it from the way he writes) is that all - or even most - of the accepted traditional songs were actually 'fakesong'.