Backbone

1. In general usage, the "backbone" is a synonym for the spine of an animal.
2. In telecommunication, the term backbone has the following meanings:

• The high-traffic-density connectivity portion of any communications network.
• In packet-switched networks, a primary forward-direction path traced sequentially through two or more major relay or switching stations.

In packet-switched networks, a backbone consists primarily of switches and interswitch trunks.

Source: from Federal Standard 1037C and from MIL-STD-188