A high-speed line or series of connections that forms a major pathway within a network. The term is relative as a backbone in a small network will likely be much smaller than many non-backbone lines in a large network.
The generic term for LAN or WAN connectivity between subnetworks across the enterprise. Generally a conduit for traffic between multiple networks which must operate at an order of magnitude greater speed and capacity than the networks it connects. Backbones are generally bordered by either switches which consult routers or by routers.
The Internet backbone is a conglomeration of multiple networks, routing facilities, and servers that provide a multiple redundancy approach to keeping information online available and secure. Simply put, it enables back up networks to take on the load of a failed network or multiple failed networks.