A darknet is a private network that is invisible to the outside world. It can be used for filesharing, or for political messaging in countries without political freedom.
“A darknet is a private, distributed P2P file sharing network." (http://en.wikipedia.org/wiki/Darknet_%28file_sharing%29)
- "The Darknet and the Future of Content Distribution" (http://msl1.mit.edu/ESD10/docs/darknet5.pdf)