Where did the word Internet come from?

The word Internet comes from "Inter-Networking" which is simply the description applied to several "computer networks" linked together.

The concept of internetworking first appeared to enable the connection of research and academic institutions. In 1983, it had reached a point whereby individual networks could be connected securely and so that when a single component failed it did not bring down the whole network, or "net".

"Inter-network" has come to popularly refer to multiple networks not owned by one entity. It is possible to have an "Inter-network" that is not open to the public therefore forming an "Inter-network" that is not part of THE Internet.

"Intra-network" indicates multiple networks joined together but owned by one entity and closed off from any outsiders.

The Internet is not the same as the World Wide Web, the establishment of which is ascribed to Tim Berners-Lee. The web relies on the internet, but the internet does not rely on the web. The internet is the hardware underpinning the www, from cables that are occasionally inadvertently cut by trawlermen to enormous "switches" that route information.

Regarding the actual etymology of the word

There are two parts to this word: the prefix "inter-", and "net", short for "network". The prefix "inter" comes from Latin, meaning (in this context) "within a group". The word network itself comes from the origins meaning "intersecting lines", in the same sense as a network of streets; computers logically connect to each other through a series of logically interconnected lines of communication.