Source : Webster's Revised Unabridged Dictionary (1913)

Backbone \Back"bone"\, n. [2d back,n.+ bone. ]
   1. The column of bones in the back which sustains and gives
      firmness to the frame; the spine; the vertebral or spinal
      column.

   2. Anything like, or serving the purpose of, a backbone.

            The lofty mountains on the north side compose the
            granitic axis, or backbone of the country. --Darwin.

            We have now come to the backbone of our subject.
                                                  --Earle.

   3. Firmness; moral principle; steadfastness.

            Shelley's thought never had any backbone. --Shairp.

   {To the backbone}, through and through; thoroughly; entirely.
      ``Staunch to the backbone.'' --Lord Lytton.

Source : WordNet®

backbone
     n 1: a central cohesive source of support and stability; "faith
          is his anchor"; "the keystone of campaign reform was the
          ban on soft money"; "he is the linchpin of this firm"
          [syn: {anchor}, {mainstay}, {keystone}, {linchpin}, {lynchpin}]
     2: fortitude and determination; "he didn't have the guts to try
        it" [syn: {grit}, {guts}, {moxie}, {sand}, {gumption}]
     3: the series of vertebrae forming the axis of the skeleton and
        protecting the spinal cord; "the fall broke his back"
        [syn: {spinal column}, {vertebral column}, {spine}, {back},
         {rachis}]
     4: the part of a network that connects other networks together;
        "the backbone is the part of a communication network that
        carries the heaviest traffic"

Source : Free On-Line Dictionary of Computing

backbone
     
         The top level in a hierarchical {network}.  {Stub
        networks} and {transit networks} which connect to the same
        backbone are guaranteed to be interconnected.
     
        See also: {Internet backbone}.
     
        (1998-07-02)