The Backbone reference article from the English Wikipedia on 24-Apr-2004
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Backbone

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

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