WordNet sense 1 (a designated route followed by airplanes in flying from one airport to another):

skyway, air lane, flight path

WordNet sense 2 (the passages through which air enters and leaves the body):

respiratory tract

WordNet sense 3 (a duct that provides ventilation (as in mines)):

air passage, air duct

WordNet sense 4 (a commercial enterprise that provides scheduled flights for passengers):

airline, airline business