.onion
Top-level domain
.onion is a special-use top-level domain name designating an anonymous onion service, which was formerly known as a "hidden service", reachable via the Tor network. Wikipedia
Intended use: To designate an onion service reachable via Tor
Registration restrictions: Addresses are "registered" automatically by Tor client when an onion service is set up
Registry: Tor
Structure: Names are opaque strings generated from public keys