A darknet is a private virtual network where users connect only to people they trust. Typically such networks are small, often with fewer than 10 users each . In its most general meaning, a darknet can be any type of closed, private group of people communicating, but the name is most often used specifically for file sharing networks.

The term was originally coined in the 1970s to designate networks which were isolated from ARPANET (which evolved into the Internet) for security purposes. Some darknets were able to receive data from ARPANET but had addresses which didn't appear in the network lists and would not answer pings or other inquiries. The root of the name is believed to be related to the term black box, which meant a system or device whose contents were unknown. Darknets that can take information from the larger net are also known as Data Motels, a reference to an advertising slogan for the commercial insect trap Roach Motel (product), where "roaches check in, but they do not check out".

The term gained public acceptance following publication of The Darknet and the Future of Content Distribution, a 2002 article by Peter Biddle, Paul England, Marcus Peinado, and Bryan Willman, four employees of Microsoft. They argued that the presence of the darknet was the major hindrance to the development of workable DRM technologies. This term has since seen usage in major media sources, including Rolling Stone and Wired, and it is also the title of a book by J.D. Lasica.

Lucas Gonze has compared "the darknets" to "the lightnet", which is the network in which content is freely shareable and available via standard URIs. In his estimation: "lightnet" is always singular (because everything in it is interoperable and interconnected), "darknets" is always plural (because by definition they are not interoperable or interconnected). (From a comment on The Jason Boog Show: Darknets and the Lightnet.)

When used to describe a file sharing network, the term is often used as a synonym for "friend-to-friend" - both describing networks where direct connections are only established between trusted friends. However, "darknet" can also be used in a broader sense to describe any private file sharing network. The most widespread file sharing networks, such as Kazaa, are not darknets, since peers will communicate with anybody else on the network. Popular darknet software includes Nullsoft's WASTE and Freenet. The present version of Freenet, unlike typical darknets, is capable of supporting potentially millions of users using an application of small world theory.

Early versions of Apple's iTunes allowed users to specify the IP of a remote subnet and share their music with users in that subnet in a darknet-like fashion. Newer versions disable that functionality, but still allow users to stream music within their own subnet; hacks such as ourTunes allow users on the same iTunes network to download each others' music with no loss of quality.

"Darknet" can also be used to refer to the state of the internet during the early 1990s when search engines would pull up irrelevant websites, making it hard to find relevant information.