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

Internet backbone

    High-speed networks that carry
   Internet traffic.

   These communications networks are provided by companies
   such as AT&T, GTE, IBM, MCI, Netcom, Sprint,
   UUNET and consist of high-speed links in the T1, T3,
   OC1 and OC3 ranges.  The backbones carry Internet
   traffic around the world and meet at Network Access Points
   (NAPs).

   Internet Service Providers (ISPs) connect either directly to
   a backbone, or they connect to a larger ISP with a connection
   to a backbone.

   The topology of the "backbone" and its interconnections may
   once have resembled a spine with ribs connected along its
   length but is now almost certainly more like a fishing net
   wrapped around the world with many circular paths.

   [Map?]

   (1998-07-02)