Meaning of backbone

back•bone

1. the spinal column; spine.
2. strength of character; resolution.
3. something resembling a backbone in appearance, position, or function.
4. a back or bound edge of a book; spine.
5. a rope running along the middle of an awning, as a reinforcement and as an object to which a supporting bridle or crowfoot may be attached.
6. the central fore-and-aft assembly of the keel and keelson, giving longitudinal strength to the bottom of a vessel.