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

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In general usage, the backbone is a synonym for the spine of an animal. It is also slang for "courage".

In telecommunication, the term backbone has the following meanings:

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

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