Glossary
Special-use domain name
Special-use domain names are reserved so they never resolve on the public internet: .test and .example for testing and documentation, .localhost for your own machine, .local for multicast DNS, .onion for Tor, .internal for private networks.
Most are listed in IANA's Special-Use Domain Names registry, created by RFC 6761. Because nobody can register them, software can rely on them never colliding with a real website — which is exactly why you should use them instead of made-up names like .lan.