n.

1. a central cohesive source of support and stability;
   anchor, mainstay, keystone, linchpin, lynchpin
   
   • faith is his anchor
   • the keystone of campaign reform was the ban on soft money
   • he is the linchpin of this firm
2. fortitude and determination;
   grit, guts, moxie, sand, gumption
   
   • he didn't have the guts to try it
3. the series of vertebrae forming the axis of the skeleton and protecting the spinal cord;
   spinal column, vertebral column, spine, back, rachis
   
   • the fall broke his back
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;
   spine
   
   • the title and author were printed on the spine of the book
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