With all due respect to my distinguished colleague from the great State of Georgia (Whoops! This is what happens when you read transcripts of Senatorial hearings.), I can't concur. The problem is that this definition leads us down a black hole and into a singularity of circular definition. I believe that there are many things that we might do at a filksing that aren't filk; that there are songs written by filkers that are not filksongs; that there are songs written by non-filkers that are filksongs. This doesn't mean that any of these things are bad or wrong, it simply means that a thing is what it is and we shouldn't confuse it with something else.
I think that if we're going to define "filk", we need something along these lines that is less inclusive than some of the definitions that I've seen going around:
"Filk is the folk music of science-fiction fandom including both original material and parodies. Generally, filk will have science fiction or fantasy themes, but may also deal with areas of broad fannish interest such as space flight, computers, and fandom itself."
Now I'm a filker, but not every song that I write is filk. Weird Al is not a filker, but songs like Yoda are pretty clearly filk. And if decadent_dave sings songs by Stan Rogers at a filksing, that doesn't mean that this isn't a filksing, nor does it mean that Stan's music was a filksong.
A secondary definition is that filk refers to the folk music of a particular fandom. Thus, Starsky and Hutch fans might write Starsky and Hutch filk, but that wouldn't normally be what we were referring to as "filk" with no qualifier.
So am I completely off the reservation or am I making sense?