• back‧bone
• 1 [countable] the row of connected bones that go down the middle of your back [= spine]
• 2

   the backbone of something

  the most important part of an organization or group of people:
• Farmers are the backbone of this community.
• 3 [uncountable] courage and determination:
• Stuart doesn't have the backbone to be a good manager.

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.