Synonyms for Backbone (same or very similar meaning)
WordNet sense 1 (a central cohesive source of support and stability):
anchor, keystone, linchpin, mainstay, lynchpinWordNet sense 2 (the series of vertebrae forming the axis of the skeleton and protecting the spinal cord):
back, spine, rachis, spinal column, vertebral columnWordNet sense 3 (fortitude and determination):
grit, gumption, guts, moxie, sandFrom the ODE community, based on WordNetadd/edit