Backbone
A portion of a computer network that is capable of carrying the majority of traffic on the network at high speeds. The backbone often connects large networks or companies together. The first Internet backbone was made between UCLA and SLI on October 29, 1969 and today the Internet consists of dozens of large backbones that carry the majority of traffic throughout the world. In the United States, many of these backbones are run by telecommunication companies such as AT&T, Bell South, Congent, Qwest, Level 3, MCI/Worldcom, Sprint, and Time Warner.
In the above image, done by Donna Cox and Robert Patterson of the NCSA, is a visual representation of what an Internet backbone in the United States may look like. This picture is of the NSFNET T1 backbone and the volume of traffic going over that network backbone for the month of September 1991. The color purple represents zero bytes of data, and the white represents 100 billion bytes of data.
Without a network backbone, you would need to pass through dozens or even hundreds of different computer and routers in order to get access to the computer storing what you need. This is why when a network backbone goes down your Internet connection speed can be drastically reduced or may not even be able to go through to certain places in the world.
Also see: Network definitions, Network neutrality, NSFNET, NSP, Router, Trunk