An Internet top-level domain TLD used exclusively for Internet-infrastructure purposes.
The .arpa TLD was originally intended to be a temporary measure to facilitate the transition to the Domain Name System.
The ARPANET was the predecessor to the Internet established by the United States Department of Defense Advanced Research Projects Agency (ARPA), and when the Domain Name System was introduced in 1985, ARPANET host names were initially converted to domain names by adding .arpa to the end.
Hostnames in other networks were also sometimes converted to pseudo-domain-style addresses by adding endings such as .uucp and .bitnet, though these were never added to the Internet root as formal TLDs.
Domain names of this form were rapidly phased out by replacing them with domain names using the other, more informative, TLDs.
However, deleting .arpa once it had served its transitional purpose proved to be impractical, because in-addr.arpa was used for reverse DNS lookup for IP addresses.
For example the IP address 145.97.39.155 is mapped to a host name by issuing a DNS query for the PTR record for the special host name 155.39.97.145.in-addr.arpa.