Onion routing is a method of using multiple layers of encryption to secure and anonymize data transfers over a network. It can be used for any network transfers but has become synonymous with the dark web due to its use by TOR (The Onion Router) to anonymize both nefarious and legitimate activity on the dark web. There are approximately 1,000 TOR proxy servers on the internet.
Onion Routing employs multiple layers of encryption, using different keys at various points on a data transfer to protect data. These are likened to layers of an onion. The encryption layers are unpeeled at the final destination. This is the origin of the name Onion Routing, TOR, and the use of Onionland as a synonym for the Dark web and the name of a prominent dark web search site.
See Onionland and TOR (The Onion Router).