The Collaborative International Dictionary of English v.0.48:

Backbone \Back"bone"\ (b[a^]k"b[=o]n`), n. [2d back, n. + bone.]
   [1913 Webster]
   1. The column of bones in the back which sustains and gives
      firmness to the frame; the spine; the vertebral or spinal
      column.
      [1913 Webster]

   2. Anything like, or serving the purpose of, a backbone.
      [1913 Webster]

            The lofty mountains on the north side compose the
            granitic axis, or backbone of the country. --Darwin.
      [1913 Webster]

            We have now come to the backbone of our subject.
                                                  --Earle.
      [1913 Webster]

   3. Firmness; moral principle; steadfastness.
      [1913 Webster]

            Shelley's thought never had any backbone. --Shairp.
      [1913 Webster]

   To the backbone, through and through; thoroughly; entirely.
      "Staunch to the backbone." --Lord Lytton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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, backbone, linchpin,
         lynchpin]
    2: fortitude and determination; "he didn't have the guts to try
       it" [syn: backbone, 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, backbone,
       back, rachis]
    4: the part of a book's cover that encloses the inner side of
       the book's pages and that faces outward when the book is
       shelved; "the title and author were printed on the spine of
       the book" [syn: spine, backbone]
    5: the part of a network that connects other networks together;
       "the backbone is the part of a communication network that
       carries the heaviest traffic"

The Free On-line Dictionary of Computing (19 January 2023):

backbone network
backbone

    In a hierarchical network, a top-level network
   that carries network traffic between the mid-level networks
   and stub networks that connect to it.

   The largest backbone network is the Internet backbone.

   (2017-12-02)