Fen--what?

[gryfeathr]

There is a term that I have run across only in the past month or so; one that strikes me with an almost-understanding.

So, I assume someone knows a good definition: What the heck is 'fen'? (I've seen this used in correlation with yaoi fandom, and fandom in general--it seems rather meta, but does anyone else know how it started/what it really means?)