The standard networking name (hostname) for "this computer," the computer you are working in. In a TCP/IP network, localhost is the name of the machine at the loopback address 127.0.0.1, which for testing purposes sends signals back to the computer instead of out to the network.
Reserved Domain Names
The IETF has reserved the top-level domains (TLDs) ".localhost," ".invalid," ".example" and ".test" for private testing. Should any URLs with these domains accidentally enter the global Internet, they would be dropped. See loopback address