In hypertext parlance, a Jane's Space is a part of a hypertext that you can't find in the usual, link-following way. A Web page that's not linked to your site and that's hidden from the search engines is a Jane's space; you can only get there if you happen to know the URL. Jane's spaces are named for Jane Yellowlees Douglas (it's a long story)
I recently wrote a small program that scans Storyspace documents, looking for spaces with text but no inbound links. Of 28 published hypertexts, at least 16 appear to have Jane's spaces. I knew some of these, but the overall total seems extroadinarily high.