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"

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "backbone":
   advocate, alpenstock, arm, athletic supporter, back, backing,
   bandeau, bearer, bottom, bra, brace, bracer, bracket, brassiere,
   buttress, cane, carrier, cervix, chutzpah, corset, courage, crook,
   crutch, determination, firmness, fortitude, foundation garment,
   fulcrum, gameness, girdle, grit, guts, gutsiness, guttiness, guy,
   guywire, hardihood, heart, heart of oak, intestinal fortitude,
   jock, jockstrap, mainstay, maintainer, mast, mettle,
   mettlesomeness, moxie, neck, nerve, pillar, pith, pluck,
   pluckiness, prop, purposefulness, rachis, reinforce, reinforcement,
   reinforcer, resoluteness, resolution, resolve, rest, resting place,
   rigging, sand, shoulder, shroud, spinal column, spine, spirit,
   sprit, spunk, spunkiness, stability, staff, stamina,
   standing rigging, stave, stay, staying power, stick, stiffener,
   stout heart, strength, strengthener, sturdiness, support,
   supporter, sustainer, toughness, true grit, upholder, vertebrae,
   vertebral column, walking stick, will



The Free On-line Dictionary of Computing (30 December 2018):

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)