A network of networks is known as an "internetwork" or more commonly, the "Internet." The Internet is a global system of interconnected computer networks that use standardized communication protocols to link devices worldwide. It is a vast network infrastructure that allows various networks, ranging from small local networks to large global networks, to communicate and share information.
ARPANET stands for Advanced Research Projects Agency Network. ARPANET was one of the earliest packet-switched networks and played a key role in the development of the Internet. It was funded by the United States Department of Defense's Advanced Research Projects Agency (ARPA, now known as DARPA), and it became operational in the late 1960s.