".test" is recommended for use in testing of current or new DNS related code.
".example" is recommended for use in documentation or as examples.
".invalid" is intended for use in online construction of domain that are sure to be invalid and which it is obvious at a are invalid.
The ".localhost" TLD has traditionally been statically defined in DNS implementations as having an A record pointing to the back IP address and is reserved for such use. Any other use conflict with widely deployed code which assumes this use.
