I found this definition when I did a search on "filk":

"There are filk songs about science fiction and stories. There are filk songs about characters from those stories. There are filk songs that are their own science fiction of fantasy stories. There are filk songs about NASA. There are filk songs about science fiction fans. There are filk songs about the Society for Creative Anachronism. There are filk songs about computers. And, of course, there are filk songs about other filksongs, or about filkers, or about the overall phenomenon of filk.

Filk songs can be silly or serious or in between. Some of them are downright morose, others positively giddy. They can be clever executions of cute ideas, or serious literary/poetic works.

While many people think first of parodies when they think of filk, much filk is actually written with original tunes rather than as parodies.

Filk is what science fiction fans and medievalists have written to sing to each other."

Sorry, I didn't copy the URL, so I can't give proper reference.