The origin of the Internet 2

The term "Internet " was adopted in the first RFC published on the TPC protocol as an abbreviation of the term internetworking and the two terms were used interchangeably.
As interest in widespread networking grew and new application for it were developed, the Internet's technologies spread throughout the rest of the world.The network-agnostic approach in TCP/IP meant that it was easy to use any existing network infrastructure.
Many sites unable to link directly to the Internet started to create simple gateways to allow transfer of email ,at that time the most important application.Sites which only had intermittent connections used UUCP(an abbreviation of Unix-to-Unix Copy) or FidoNet(a worldwide computer network that is used for communication between bulletin board systems) and relied on the gateways between these networks and the Internet. Some gateway services went beyond simple email peering, such as allowing to FTP(File Transfer Protocol) sites via UUCP or email.

Reference:en.wikipedia.org/wiki/History_of_the_Internet#Transition_towards_the_Internet