Backbone: It is a larger transmission line which carries data gathered from smaller lines which interconnect with it.
1) At local level, a backbone is a line or set of lines which local area networks join to for a wide area network connection or within a local area network to span distances proficiently (for illustration, between buildings).
2) On Internet or other wide area network (WAN), a backbone is a set of paths which local or regional networks join to for long-distance interconnection. The connection points are termed as network nodes or telecommunication data switching exchanges (abbreviated as DSEs).