"I think the problem here is that we keep using the word 'definition' for something which can't be defined."
It can and has been defined - what can't be defined is what is happening at folk clubs today.
At the beginning of the twentieth century a group of people accumulated a large number of songs, largely from the rural population.
They were closely related to each other in form and many of them appeared in different parts of the country in different versions - they were also found in Ireland, the United States and Canada - all over the English speaking world - the older ones, the ballads, also turned up in other languages.
The vast majority of them were anonymous and were claimed by the people who sang them to be 'local' - Norfolk, Aberdeenshire, Clare, wherever - most of the singers we recorded in Ireland were totally unaware that many of their songs had English or Scots variants.
The old claim that "I know a folk song when I hear it" is still a valid one - though no longer applicable in the clubs.
A genre of songs were found, somebody a name, which was more or less agreed on, and in 1954, a group of researchers got together and attempted to fill out a usable definition from the work that had been carried out so far in Britain, Ireland, America, Canada.
Around the end of the 1950s, another group of people got together and launched a Revival based on the previous body of songs and another five year campaign of collecting by the BBC, encouraged by Alan Lomax.
Part of that revival was inspired by similar work carried out in the United States by The Library of Congress.
We have a specific and identifiable body of songs, we have an attempt to identify that body of songs socially and culturally and up to relatively recently we have had a consensus on what those songs are - that is our 'definition'.
The only thing that ahs changed is that we now have a revival that no longer accepts any definition and has turned folk sessions into singing nights, we call them 'Singing Circles' in Ireland.   
Jim Carroll