The “dark web” is a journalistic (AFAICT) coinage arising from a confusion between two terms: a “darknet” (antonym: “clearnet”) is a network that traverses the Internet in such a way that the Internet’s routing infrastructure can’t see the ultimate destination (Tor, but also DN42, your work VPN, etc.); the “deep web” is the part of the web that’s not accessible to search engines and thus only really used by those who already know it exists (Tor hidden services, pirate libraries, private torrent trackers, but also every other registration-gated forum and arguably even Facebook Marketplace—the term is from a more innocent time with much fewer walled gardens).