Hub and Spoke Dossier
A hub is a node that other nodes depend on — and dependence, not link count, is the defining trait.
Start with the plain picture. A network is points and links: cities and routes, devices and cables, people and relationships. Most points are interchangeable — remove one and the network barely notices. A few are load-bearing: remove one and trips lengthen, flows reroute, and whole regions can lose contact. The load-bearing points are hubs.
The obvious test — count the links — is a real measure, but an incomplete one. Degree tells you how much of the network sits within one step of a node. It does not tell you whether those neighbors lead anywhere. A node can be the most connected point in a dead end, while a modest three-link node holds two halves of the map together.
Position is the deeper trait. A hub collects: flows from many points converge on it. It redistributes: arrivals are sorted and sent onward. It abbreviates: any two of its neighbors are two steps apart through it, however far apart they lie on the map. These are jobs, not sizes, which is why small nodes sometimes do them.
The confusion between many links and central matters because the consequences differ. If a hub is merely popular, its failure inconveniences its neighbors. If a hub is structural, its failure splits the network. The measures in this dossier — degree, closeness, betweenness — exist to tell the two cases apart before the failure does.
A useful habit is to ask of any important-looking node: important to whom, and for what? The answer picks the measure, and the measure picks the node. There is no context-free most central point — only nodes that are central for a stated reason.
Further reading