Definition of "Backbone" :

noun: the part of a network that connects other networks together

"The backbone is the part of a communication network that carries the heaviest traffic."

noun: the part of a book's cover that encloses the inner side of the book's pages and that faces outward when the book is shelved

noun: fortitude and determination

noun: the series of vertebrae forming the axis of the skeleton and protecting the spinal cord

noun: a central cohesive source of support and stability