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Synonyms for Backbone (same or very similar meaning)

WordNet sense 1 (a central cohesive source of support and stability):
anchor, keystone, linchpin, mainstay, lynchpin

WordNet sense 2 (the series of vertebrae forming the axis of the skeleton and protecting the spinal cord):
back, spine, rachis, spinal column, vertebral column

WordNet sense 3 (fortitude and determination):
grit, gumption, guts, moxie, sand

From the ODE community, based on WordNetadd/edit