back·bone
 /ˈbækˌboʊn/  noun  ,  pl   -bones
  1 [  count  ] : the row of connected bones that go down the middle of the back and protect the spinal cord — called also spinal column, spine
  2 [  count  ] : the most important or strongest part of something
   She is the backbone of the family.
  3 [  noncount  ] : strength and courage
   He showed some backbone by refusing to compromise his values.