The Collaborative International Dictionary of English v.0.48:

internet \in"ter*net\ ([i^]n"t[~e]r*n[e^]t), n.
   A large network[3] of numerous computers connected through a
   number of major nodes of high-speed computers having
   high-speed communications channels between the major nodes,
   and numerous minor nodes allowing electronic communication
   among millions of computers around the world; -- usually
   referred to as the internet. It is the basis for the
   World-Wide Web.
   [PJC]

WordNet (r) 3.0 (2006):

internet
    n 1: a computer network consisting of a worldwide network of
         computer networks that use the TCP/IP network protocols to
         facilitate data transmission and exchange [syn: internet,
         net, cyberspace]

The Jargon File (version 4.4.7, 29 Dec 2003):

Internet
 n.

    The mother of all networks. First incarnated beginning in 1969 as the
    ARPANET, a U.S. Department of Defense research testbed. Though it has been
    widely believed that the goal was to develop a network architecture for
    military command-and-control that could survive disruptions up to and
    including nuclear war, this is a myth; in fact, ARPANET was conceived from
    the start as a way to get most economical use out of then-scarce
    large-computer resources. Robert Herzfeld, who was director of ARPA at the
    time, has been at some pains to debunk the ?survive-a-nuclear-war? myth,
    but it seems unkillable.

    As originally imagined, ARPANET's major use would have been to support what
    is now called remote login and more sophisticated forms of distributed
    computing, but the infant technology of electronic mail quickly grew to
    dominate actual usage. Universities, research labs and defense contractors
    early discovered the Internet's potential as a medium of communication
    between humans and linked up in steadily increasing numbers, connecting
    together a quirky mix of academics, techies, hippies, SF fans, hackers, and
    anarchists. The roots of this lexicon lie in those early years.

    Over the next quarter-century the Internet evolved in many ways. The
    typical machine/OS combination moved from DEC PDP-10s and PDP-20s,
    running TOPS-10 and TOPS-20, to PDP-11s and VAXen and Suns running 
    Unix, and in the 1990s to Unix on Intel microcomputers. The Internet's
    protocols grew more capable, most notably in the move from NCP/IP to TCP/
    IP in 1982 and the implementation of Domain Name Service in 1983. It was
    around this time that people began referring to the collection of
    interconnected networks with ARPANET at its core as ?the Internet?.

    The ARPANET had a fairly strict set of participation guidelines --
    connected institutions had to be involved with a DOD-related research
    project. By the mid-80s, many of the organizations clamoring to join didn't
    fit this profile. In 1986, the National Science Foundation built NSFnet to
    open up access to its five regional supercomputing centers; NSFnet became
    the backbone of the Internet, replacing the original ARPANET pipes (which
    were formally shut down in 1990). Between 1990 and late 1994 the pieces of
    NSFnet were sold to major telecommunications companies until the Internet
    backbone had gone completely commercial.

    That year, 1994, was also the year the mainstream culture discovered the
    Internet. Once again, the killer app was not the anticipated one ?
    rather, what caught the public imagination was the hypertext and multimedia
    features of the World Wide Web. Subsequently the Internet has seen off its
    only serious challenger (the OSI protocol stack favored by European
    telecoms monopolies) and is in the process of absorbing into itself many of
    the proprietary networks built during the second wave of wide-area
    networking after 1980. By 1996 it had become a commonplace even in
    mainstream media to predict that a globally-extended Internet would become
    the key unifying communications technology of the next century. See also 
    the network.


The Free On-line Dictionary of Computing (30 December 2018):

Internet

   

   1. With a lower-case "i", any set of networks interconnected
   with routers.

   2. With an upper-case "I", the world's collection of
   interconnected networks.  The Internet is a three-level
   hierarchy composed of backbone networks, mid-level networks,
   and stub networks.  These include commercial (.com or .co),
   university (.ac or .edu) and other research networks (.org, .net)
   and military (.mil) networks and span many different physical
   networks around the world with various protocols, chiefly the
   Internet Protocol.

   Until the advent of the web in 1990, the Internet was almost
   entirely unknown outside universities and corporate research
   departments and was accessed mostly via command line interfaces
   such as telnet and FTP.  Since then it has grown to become a
   ubiquitous aspect of modern information systems, becoming highly
   commercial and a widely accepted medium for all sort of customer
   relations such as advertising, brand building and online sales
   and services.  Its original spirit of cooperation and freedom
   have, to a great extent, survived this explosive transformation
   with the result that the vast majority of information available on
   the Internet is free of charge.

   While the web (primarily in the form of HTML and HTTP) is the
   best known aspect of the Internet, there are many other
   protocols in use, supporting applications such as electronic
   mail, chat, remote login and file transfer.

   There were 20,242 unique commercial domains registered with
   InterNIC in September 1994, 10% more than in August 1994.
   In 1996 there were over 100 Internet access providers in the
   US and a few in the UK (e.g. the BBC Networking Club,
   Demon, PIPEX).

   There are several bodies associated with the running of the
   Internet, including the Internet Architecture Board, the
   Internet Assigned Numbers Authority, the Internet
   Engineering and Planning Group, Internet Engineering
   Steering Group, and the Internet Society.

   See also NYsernet, EUNet.

   The Internet Index (http://openmarket.com/intindex) -
   statistics about the Internet.

   (2015-03-26)