the top-level, or primary, routers and telecommunications circuits interconnecting all of the major computer networks that make up the Internet.
Well actually there are many backbones. These are extremely fast connections(T3 and higher) crisscrossing the globe from one major metropolitan area to another. They are operated by global telecommunication companies and all local ISP(Internet service providers) are connected to these backbone connection through access points.
a super high capacity communications network