In reply to Steve, I was using the term 'origin' as a shorthand for that evolutionary process which he refers to. I was trying to distinguish between that meaning and the other stylistic meaning.

In reply to Phil, I am using 'style' in a very broad sense. The classification of music by genre is based principally on what it sounds like. If you play people different pieces of music and ask them to describe them, they will call them 'folk', 'pop', 'jazz', 'classical' or whatever based on what they sound like, rather than a detailed analysis of form or structure, or whether they have been through some evolutionary process. This is why, although "The Foggy Dew" is a 'folk song', when sung by Peter Pears it isn't 'folk'.

Birdwatchers use the term 'jizz' to describe the overall appearance of a bird which allows it to be identified from its outline, flight pattern or other broad characteristics, rather than from particular details. The same approach applies to classifying music - I may not be able to define 'folk' or 'jazz' or other musical genres, but I have a broad idea of what they sound like which is based on how these terms are generally understood, not just my personal interpretation.

Of course, whereas birds can be positively identified as belonging to one species or another, music isn't like that. There are some pieces which defy categorisation, and for these in particular deciding whether to put them into the 'folk' pigeonhole will be far more subjective, and probably influenced far more by individual taste and preferences. This is why there is such an argument about it, since individuals may quite reasonably have different opinions about where the boundaries should lie.

To say that 'folk' is what can be performed at folk clubs is a circular argument. Nevertheless it is the reality. 'Folk' when used as a label for a genre can be no more than an imprecise idea, it is not a definition.