USA: Congressional Research Service: Dark Web
14 July 2015
"The furthest corners of the Deep Web, segments known as the Dark Web, contain content that has been intentionally concealed. The Dark Web may be used for legitimate purposes as well as to conceal criminal or otherwise malicious activities. It is the exploitation of the Dark Web for illegal practices that has garnered the interest of officials and policy makers."
See:
Dark Web (pdf)
See also:
How Did The FBI Break Tor? (Forbes, July 2014, link):
"Global law enforcement conducted a massive raid of the Dark Web this week."