What is the meaning of backbone?

noun

• A central cohesive source of support and stability.
  • usage: "faith is his anchor"; "the keystone of campaign reform was the ban on soft money"; "he is the linchpin of this firm"
  • synonyms: anchor, mainstay, keystone, linchpin, lynchpin
• Fortitude and determination.
  • usage: "he didn't have the guts to try it"
  • synonyms: grit, guts, moxie, sand, gumption
• The series of vertebrae forming the axis of the skeleton and protecting the spinal cord.
  • usage: "the fall broke his back"
  • synonyms: spinal column, vertebral column, spine, back, rachis
• 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.
  • usage: "the title and author were printed on the spine of the book"
  • synonyms: spine
• The part of a network that connects other networks together.
  • usage: "the backbone is the part of a communication network that carries the heaviest traffic"