Hub  is an airport or city in which an airline has a major presence, routing many flights into and out of that city.

Other definition:
Hub is airport at which an airline has major operations and connecting flights to smaller destinations.

an airport or city in which an airline has a major presence and many flights to other destinations. As an example, Delta has a hub in Atlanta. Many carriers use the hub-and-spoke system to maximize profits by keeping the aircraft in the air as much as possible. Flights to the hub are many, and from there flights to many other destinations are scheduled
Other definition:
Hub is an airport in which the majority of an airline's flights arrive, depart, and connect with each other. An airline may have one or more hubs.

Other definition:
Hub is an airport or city which serves as a central connecting point for aircraft, trains or buses from outlying feeder airports or cities.

List of books: Hub