An airline hub is an airport that an airline uses as a transfer point to get passengers to their intended destination.^{[citation needed]} It is part of a hub and spoke model, where travelers moving between airports not served by direct flights change planes en route to their destinations.^{[citation needed]} Many hubs of the airlines are also situated at airports in the cities of the respective head offices.^{[citation needed]}

Some airlines may use only a single hub, while other airlines use multiple hubs. Hubs are used for both passenger flights as well as cargo flights.^{[citation needed]}

Many airlines also use focus cities, which function much the same as hubs. Airlines may also use secondary hubs, a non-technical term for large focus cities.^{[citation needed]}

A fortress hub is an airport where a single airline's share of flights is at or above the monopoly standard of 70 percent of flights in and out of the hub. For example, in 2010 US Airways occupied 85 (plus 1 shared with Lufthansa) out of 97 total gates and accounted for approximately 90% of passenger traffic at Charlotte/Douglas International Airport. Another example is at Detroit (DTW), a Delta Air Lines hub. New entrants, such as Spirit Airlines at Detroit (DTW), AirTran at Atlanta (ATL), and Vanguard at Dallas-Fort Worth (DFW), allege to have been the target of exclusionary practices by the dominant carrier. Some observers argue that the existence of such hubs can stifle competition; ProAir's battle with Northwest when it briefly flew out of Detroit City Airport is often cited as an example. Northwest was able to out compete the short-lived discount carrier by matching its fares and offering more frequent flights. Although these competitive measures have nothing to do with hub status per se, they are indicative of the measures a hub airline will take to defend its preferred position at a hub airport.

Leonardo di ser Piero da Vinci (Italian pronunciation: [leoˈnardo da ˈvintʃi]  pronunciation (help·info); April 15, 1452 – May 2, 1519, Old Style) was an Italian Renaissance polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist, and writer whose genius, perhaps more than that of any other figure, epitomized the Renaissance humanist ideal. Leonardo has often been described as the archetype of the Renaissance Man, a man of "unquenchable curiosity" and "feverishly inventive imagination". He is widely considered to be one of the greatest painters of all time and perhaps the most diversely talented person ever to have lived. According to art historian Helen Gardner, the scope and depth of his interests were without precedent and "his mind and personality seem to us superhuman, the man himself mysterious and remote". Marco Rosci points out, however, that while there is much speculation about Leonardo, his vision of the world is essentially logical rather than mysterious, and that the empirical methods he employed were unusual for his time.